is this unforgivable in a friendship?

You’re just a friend it’s easy to cut you off. He is her husband they are emotionally, financially, pragmatically linked in a way that is far harder just to block. Not to mention the happiness and financial security of her children. I bet she wishes all it would take would be to block him on a few WhatsApp groups. What you have done for a bit of a thrill has likely ruined her life.

Like other posters , i genuinely feel that you are enjoying the drama in this situation. Block and move on .

You are culpable because you enabled him - you let him message you when you should have shut that down and blocked him. You're not responsible for his actions, but you are responsible for how you responded to that. You're now reaping what you've sown.

He's obviously a lying cheating scumbag, which you seem to be oblivious to, but your own actions are far from pure in motive. A decent friend would not have allowed any of that and stopped it (by withdrawing and blocking him) but you didn't.

You don't know what's been said and that he's come out of it smelling of roses. Maybe she's read all the messages and knows a lot of it was driven by him. She'd still be reasonable to cut you off.

He doesn't care for you. Not really. Or he would be with you. There is nothing stopping that. So block him and move on. You deserve to be treated better.

His wife's actions are completely justified. You aren't just some other woman, you are her friend who broke her trust.

The wife and husband's marriage status has absolutely nothing to do with you. Whether she stays or leaves is her decision. Move on.

Your actions aren't right. It doesn't matter if it was like a drug. You were friends with that woman and betrayed her repeatedly. A good friend would shut it down and block from the very beginning. You've put yourself first with the way you chose to act and now have to suffer the consequences.

Be honest are you disappointed that she hasn't thrown him out because then you could have had him?

Obviously anyone with a grain of self respect would end a friendship with someone over this.

Just to temper some of the responses on here, I'd like to point out:
As her partner, his betrayal towards her is far greater than yours was. His ties to her are stronger than yours were. Your actions were more about liking him, a lot of his were about disliking her.
But as her partner she's more invested in that relationship and has more to gain by trying to make it work. Maybe she doesn't want to, you've been cut out so you don't know that.

I'd never want to see or talk to you again if I was her but I can understand why you feel you are being scapegoated

@MissScarletInTheBallroom Righto. So you don’t feel any loyalty to your friends? Any obligation to not shit all over them? The rubbish that OW spout about “not being the one who made promises” is bad enough but when it’s your friend? Jesus. I cannot comprehend the mental gymnastics you must go through to justify being such a throughly awful human being.

Wow I think she is right to have cut you off and it sounds like the damage was done in the first part of your post when you glaze over the bit about how you ended up having feelings for one another!!! Your post focuses on the wrong parts of this story.. it’s the lead up to you having feelings that is important and that is where you have behaved really really badly, as has he. If you were a good friend at that point you would have either cut him off or you would have told your friend that her partner is messaging you etc. I can see why she cannot have you in her life…

I think your best option now is to move on from all of them and not fan the flames whatever happens between them.

You are both as bad as each other. You both admitted feelings, ok we can’t help how we feel however all contact should have stopped. He still messaged but you didn’t shut him down, if you were truly loyal to this friend you would have told him do not message me and blocked him but you didn’t you entertained him. You assume he’s come out smelling of roses, why? The wife could have found messages and as your both playing a role in the conversation your both to blame.

Because she is loving the attention.

You're getting a hard time on here OP.

You should have blocked your friend's husband so he couldn't contact you.

But he's the married one, this is on him.

Well yes, of course it is unforgivable. And you know that. For me, the partners/husbands of friends are immediately placed in “brother” territory in my head, as in, I would never, ever go there, or even think about it. Same for my husbands friends.
If any of them are ever inappropriate (and yes, it happens. Some men are just fucking sleezy) they are shut down, brutally if necessary, because I do not find it flattering or an ego boost for them to assume I’m the kind of morally bankrupt person who would betray a friend. Or an acquaintance. Or anyone.

Grow a spine and learn how to be a better friend and to protect yourself from this shit, because like you say, it’s not uncommon. But they are always, always just out for a shag. Do you want to sacrifice your friendships to be some saddos ego boost?

The wife will be reeling from all this, because she trusted both of you. It says a lot about you that your only thought for her is why she hasn’t left him? She might yet but for now, her world has imploded and you took part in that. Revolting behaviour, from both of you.

In answer to your AIBU question, no you aren't. It seems that you've been laid with 100% of the blame for her marriage (potentially) falling apart when he (if what you say is true) is at least 50% of the problem.

You need to shoulder some of the blame even if you didn't feel you encouraged him (but you have encouraged him because you told him you had feelings for him and you never blocked him).

Usually on MN, PPs comment to a woman who's facing marriage problems because their DH has a wandering eye is...'the OW never made vows to you OP, he is the problem, not her'.

So, in that respect I do see why you feel you've been unfairly treated.

But in answer to the title of your OP, yeah YABU! It's never acceptable behaviour to encourage a friends husband! I'm not surprised she's blocked you; you've hardly been a friend to her.

YABU

This puts it well.

you are enjoying the drama and I suspect half hoping he’s going to turn up on your doorstep.

playit through in your head…he turns up you end up with blended family, guilt, traumatised kids and in 5 years. Time he’s doing the same to you. You don’t get her life you get half a life.

you should have shut him down. Grow up and find some new friends.

There are some harsh answers here.
Yes, you shouldn't have admitted how you felt. Yes, you should have blocked him.
But you didn't act on your feelings and that's the really important thing here. Now he's the one sending the messages and somehow you're being treated as the one to blame.
Personally I'd go to the wife and tell her unequivocally that nothing happened. That might make her feel a teeny weeny bit better as she's probably feeling pretty tortured right now.
I'd then block him forever. He's a lying manipulative bastard.

You act as though you aren't doing anything wrong. Come on now, at least be honest with yourself. You could have easily blocked him, but you didn't, because you love the attention and the drama.

You can’t honestly think you can still be friends with her?

Look at it this way: if she knows the actual truth, or even just his version of it, she will want absolutely nothing to do with you - regardless of who is more culpable (you, the husband, or both).

She is unlikely to end her marriage over what you say is just some messages. But what she would be likely to do is instruct him to cut you out of his life, and of course she’ll want to do the same.

You say he must have told her something untrue… why? What has really happened is more than enough justification for her actions.

Unfortunately you played with fire when you entertained the possibility of feelings with someone else’s husband. You were always going to end up getting burned.

is this unforgivable in a friendship?
I'm a bit confused. You can't have thought it's ok to come on to someone's husband and have them want to be your mate still. So what does this title mean? Of course it's unreasonable to keep getting (drunk?) messages declaring feelings for you and not shut him down.

You've supposedly moved away, so just block his number and get on with your own life. How they choose to work on their relationship is not your business.

I’d tell her if you’re the one getting all the blame in a friendship group. As it’s clear he’s not going to suddenly leave his wife for you, and it’s not fair to you he’s messaging now and then, preventing you from moving on. I’d make it known to the mutual friends what happened and make it clear it was him who made the first move and you never acted on feelings. We can’t help who we fall for, but we can stop those feelings so they don’t hurt others, and that’s what you’ve done. Carry on with your life and forget this guy

I’d block you too, you are not innocent in all this. Why didn’t you block him ?

I was a single mum for 5 years, because my ex cheated , all my friends partners were completely off limits .

... but I'm being truthful about a situation and a position that I don't think gets shared too much - but probably is more common that people realize.

Love that you think you're doing some kind of public service.

Most adult women know this happens all the time. You'd just have to be very naive and very self-indulgent to fall for it,