To feel uncomfortable with this message exchange

OP says that since she has been with her partner this woman has been in a relationship, until recently.

If her partner and this woman have known each other since teens you have no idea of their sexual history. They may very well have had some sexual relationship in the past.
So you cannot say "they could have got together previously and they didn't " with any degree of certainty.

Yeah that's worrying but tread carefully here.. you don't want to push them together.
If a man wants to cheat you can't really stop him. And even if you do manage to, what are you left with? Someone you had to win round from straying??
Hopefully he will make the right choice.
You are carrying his baby. You love him and you have a real relationship.
This girl may be pretty... but she's just going through a break up and wants a bit if an ego boost from her male friend complimenting her. Even if you didn't exist at all and he was free to do as he wanted I don't think this relationship would be a good idea and work. They are both just enjoying a bit of attention.. they live very far away from each other,barely see each other, don't have much in common. She's just had her ego bruised by a breakup and he's just enjoying being needed by a pretty girl. It's not real.
I'd hope very much that he knows that.

I'd not let on I'd seen the messages. And I'd not try and stop him from going to the party. But I might seriously have a chat with him before he goes. I'd let him know that despite being pregnant I'm not a vulnerable person and I don't need to beg for attention and that if he acts in a way which is disrespectful to me whilst he's at this party, I will be ending the relationship with him. He wouldn't be having his cake and eating it on my watch. Either he wants to pursue nonsense fantasy or he wants to work on building a loving family with me. I'd say that I don't mind him being friends with women at all.. and I understand this girl is an old friend and needs support right now... but I can also see that this is a situation in which there may be opportunity for him to get his ego massaged by being needed amd it nay lead to him acting inappropriately. I'd want him to know that if he does that I wouldn't be tolerating it and he'd be leaving. So he should bare that in mind before he attends this event and have a think about how he is going to act.

The messages ARE a bit inappropriate, but I'm going to offer an alternative view in that if htey have been friends since they were teenagers a) they could have got together previously an they didn't and b) the relationship will be different and it may well be that it's about offering the sort of support that actually often comes between friends who are same sex. So if a friend broke up with her BF and was devastated, I can easily imagine myself saying to her, "he's crazy. you are the best thing that could have happened ot him, I can't believe he's chosen someone else."

I say this as someone who also has a very close male friend who I only see occassionally and who I have known since I was a pre-teen. He's sort of like a cousin now - not quite a sibling, but a cousin who you really get on with and consider a friend? When he and his wife divorced, I definitely supported him in a way thhat was similar to how I'd have supportd him when we were teenagers or how I'd support a woman and while I don't think any conversations crossed any lines, a couple of times I did think to myself, "mmm, this could come across oddly if anyone else was listening" and almost had to remind myself that because we ARE different sexes, sometimes you do have to take a slightly different approach.

Similarly, when I lost a parent, he turned up in the same way that my best female friends did/would have (most of them are in another country).

So I think it's okay to be a bit uncomfortable, but I wouldn't be going nuclear at this point.

@Notjustmeah Those messages show they definitely fancy each other.

I’d not be happy in your shoes- not many people would be! ( Men or women ) confronted with those messages.

But KarmenPQZ she follows it up with "I'd give all the money in the world for a guy like you" which is putting the ball in his court -
So the question OP is: do you trust him?

In your position I would have to speak to him about it.

As a PP says, you will learn a lot by the way he reacts

In these situations I always try to think would you send the same message to a same sex friend. The answer here is no he would not send that to a male friend. So yes he’s crossed a line and so has she. You are pregnant for god sake so how can they disrespect you like this. It needs to be either her or you because trust me from experience, you definitely don’t need her leeching on.

Whaaaaat. He’s literally saying the only way he can be with her is if he can clone himself. Ie he is 100% not able to be with her. That’s what he told her. And she replied something equally warm in a make believe world. What was she suppose to say ‘errr no thanks’. They were just texting you’re reading way too much into it unless there’s a drip feed coming.

Oh dear OP! I may get slated for saying this but even if he hasn't physically cheated, he is emotionally cheating on you which is as damaging, if not moreso.
People may grumble at you for looking at his phone but his deception of you is greater than yours is for looking. I think you may need to lose this one as giving any kind of ultimatum is likely to backfire but an ultimatum may be an option worth trying. You may need to consider how you would trust him going forwards if you choose to go down this route. Either way, i have been there and i feel for you. It hurts like hell. You are woth more than this and you deserve better.

YANBU. Breakups are upsetting and of course he wants to support his friend and she wants his support, but since he's in a relationship they both need to be extremely careful that this hand-holding doesn't turn into something else. They are not being careful. I'd suggest talking to your DP about what you've seen, and asking if he's still committed to you or hoping for a relationship with her. You'll tell a lot from the way he responds.

Some of the comments I’d see as friend trying to be supportive, like the “you’re gorgeous” type thing I’ve absolutely said to my friends when they’ve been cheated on so I don’t think that’s too out of bounds really. But the “clone” “yes please” I do think crosses a line.

So he just insults your family. Sorry but you will always be the second prize he is obviously in love with this woman.

Don't marry him even if he isn't after your money.. he can't be trusted

They are both testing the waters, and both clearly up for it.

I'd take pics of the messages, but would have a chat later saying something like you know you might be paranoid - pregnancy hormones etc - but him messaging til 4am is bugging you a bit, what were they chatting about til that time?

If he doesn't immediately offer to show you to put your mind at rest then you have your answer.

This would be crossing a line for me too. Casually putting your family down to build her self esteem isn’t what I would expect from a loving partner.
I think you should say to your DP that you know they were texting into the early hours of the morning and it’s not appropriate. Ask him how he would feel if you were texting a man at 3am!

If they wanted to be together they would have already. I don't think you need to worry, he's just trying to make her feel better. But ofcoirse it's not ok because he's in a relationship with you and should know better. You're not being unreasonable. Fess up to reading his messages and talk it out.

The flirty messages aside, he shouldn't be discussing or badmouthing your family to her, or anyone else! That would piss me off just as much as his shitty chat.

Agree with others, he's putting the feelers out. He maybe has no intention of making a move but even if it's just for an ego boost it's pathetic.

I don't know what to tell you. I have a very close friend, he is like a brother to me. He is genuinely the nicest man I know and I feel 100% comfortable to say anything to him. When I had a breakup, we also talked for several days and yes, he would absolutely tell me similar things, trying to prop up my self esteem. And at the time, the idea of having his clone by my side would have seemed very appealing. Not because I fancy him but because I feel he is the closest the most supportive person I have and gives me a great comfort knowing he is in my life. Things get blow up out of proportion at vulnerable times and I would try my best not to read too much into it.
Incidentally, this is why I have a policy of never looking at anyone's phone and never letting anyone look at mine. Not because I think there is anything to hide but because I know how strong confirmation bias can be.

Oh dear there’s no coming back from this

He has made his feelings and desires perfectly clear

You can’t un see!

Sorry this does feel as though they have both crossed a line. She’s clearly looking for some validation after being cheated on and newly single and he’s getting carried along with it. Agree this needs to be shut down before it goes any further

Absolutely not ok. He’s up till 3am sending flirty messages about how attractive she is? It’s how people behave at the start of a relationship. The issue is if you confront him, you’d have to admit you were looking at his phone, which obviously isn’t really ok.

YANBU at all. Emotional affair starting up/already started. They've both massively overstepped.

Don’t marry him. Sounds like he is after your money.

I saw one of those instagram reels the other day, saying never insult/complain about your wife to another woman. He said the majority of affairs start because a man complains about his wife and the woman in front of him immediately responds by 'bigging him up' taking his side. 'Well she doesn't appreciate you'. 'She sounds like a nag'. 'You deserve better' or in your case 'They're shallow, I'd give all the money in the world for a guy like you'

The line has been crossed. You are right to be concerned.