Very concerned about adult daughter

[Girasao]

DD is late 30s and in a very dysfunctional relationship which is unfortunately one of many. She has been seeing a married colleague of hers for at least a year now, and while I certainly don't condone this I am very worried about her. He told her from the start that he would not be leaving his wife, but yet DD keeps hoping that this will happen nontheless. Obviously this can never end well, and I find it particularly painful witness because I was in this man's wife's place for years while my DCs were growing up.
My DD was in a long term relationship with a functioning alcoholic, and she kept this hidden for a long time until she eventually reveiled the abuse she was suffering in secret. She eventually opened up and we (DS and myself) helped her feeling relieved that she could have a fresh start. I only realised later that she had
ended her previous relationship because she was hoping to start a new one with her colleague.
My DD has lost a lot of weight ovet the cause of the year. She keeps talking about her weight, and when I saw her now she brought up the topic twice and insisted on showing me her stomach insisting that she has "a bit of a belly". Both DD and I have always been naturally thin, and despite her claim she is most certainly underweight. You can see her ribs.
Since this started she is constantly on alert, constantly texting, phoning, staying engaged. We went on a family holiday and she shares a room with her DB who remarked that she was frantically texting into the middle of the night. She even told him that she felt overwhelmed by the constant expectation to respond. The last few times I saw her our visits where always overshadowed with some sort of drama. She would stress and fret and meet the man to discuss and sort things out, which meant that she would disappear for hours each time.
One thing I am not proud of: DD always asks me to keep things from her DB but I have been telling him, because I am feeling quite overwhlemed by all of this myself. I know he won't tell her that he knows, but I still feed guilty.
I just don't know what else to say and do, apart from watching her crash and burn which breaks my heart. I also feel so guilty thinking that she must have really suffered growing up with such dysfunctional parents, so went down the same route.
I don't know what I'm expecting to hear, but any word of advice or similar experience would be most appreciated.

My only advice is to stay close. One of the most important things when deciding to step away from this relationship will be the alternative support she has. If possible make sure you can offer her a room and practical help to rebuild a new life. Tell her that these things are there for her should she ever decide she wants them. If you can, try to gently paint a picture for her of what life could look like if she were free. If her life didn't have to revolve around waiting for his messages and jumping through the hoops he sets. If she freed herself to have a proper life with a man she didn't have to share.
And please stop blaming yourself. It wasn't your fault her father had an affair. You did your best then to deal with the hand you were dealt and you're doing it now. No one can do more than that.

I think your daughter is vulnerable in a very non healthy way.
Her boyfriend is exploiting her. She will waste years on this relationship.
Of course stay close.

You might not be able to change anything at all.
You might be lucky and be able to speak with the boyfriend about the unfairness to his family and to your daughter..

I would be employing a private detective. Photographs of him with his wife (and/or other flings and girlfriends) could be useful in having your daughter recognise the truth to her own fragile and exploited existance.

Would you feel able to approach him (if you know where he lives, hangs out) and firmly say “Stay away from my daughter or I will tell your wife.”
Also, “if you tell my daughter I have come to see you, I will tell your wife for that too. Stay away from her!”

Could you do that?

Thank you all. I shared my concerns with her but she has gotten more volatile over the last 5 months and lashes out at me at any opportunity. I think she'd lose it if I was to contact him directly. Saying that I only know his first name, and I know that they are working together. According to my DD their relationship has been noticed in work and they have been advised to keep a distance (which I am sure they are ignoring).
If I am honest she has always lashed out at me since she was a child, but this has been getting worse. DS has asked me for the man's surname but I genuinely don't know it, and I might not even tell DS anyway.
It probably sounds horrible DD and her partner seem as bad as each other to a degree. Once one of them tries to end it the second one freaks out and starts smothering. She has been like this in the past but this is far worse than ever before. I have just never seen her so unhealthy.

Be careful for your DS. It sounds like he's thinking of involving himself. If he knows where she works it wouldn't be that difficult for him to find this man, and the last thing you need is for him to do something that could get him into trouble. He may feel a certain level of responsibility to be 'the man of the house' and protect his sister and ease your worries. I'd make it a priority to head that off.

I agree. He is probably easily findable on linked in or the company website. From this you could find his Facebook and maybe just tell his wife? Don't say you're her mum but say you're a colleague? Give details of when they've been together etc for proof (the wife can check her phone and see that yes he did ignore calls that evening etc)

I'd be getting proof, photos etc and send to his wife, it won't be difficult to find out who he is.
Your DD sounds quite unstable and should try and be single and work on her self esteem to enable her to stay away from these horrible men.

Thank you. I don’t think there is any proof. When DD first told me about the relationship she showed me a picture of the man but according to DD none of her friends have ever seen or met him. It is very strange since they apparently go out together as well. DS asked me before if I am sure that she isn’t making the whole thing up but I don’t think she is. After all we both noticed the incessant texting and phoning when they are apart.
I don’t think DS will involve himself, and he has suggested that I ignore her whenever she tries to bring up the topic because it will just “feed her drama” but I don’t think it would be a good idea. I’m trying to get her to talk to her GP, and to hopefully talk to a counsellor but she has been putting this off for 6 months already.
It’s beyond frustrating to see her throw her life away like this, and I am also finding it hard to have to witness her suffering. DS claims that he doesn’t care any more and that it’s her own problem, but I can’t just turn off the worry.

Could you tell their work?

they might not give a shit but if one gets fired or a rumour gets out, one might quit. The less time they spend together the better. Ban him from your house too. If he gets spooked, he might back off from her totally.

It’s a horrible situation for all of you. But she’s an adult, responsible for her own actions, and there is nothing you can do to resolve it.

You need to take a step back for your own sake, and that of your DS. Let her know she can seek you out if she needs to, and don’t involve yourself any more.

All of this

We’ve already lost one daughter (stillborn), I will NOT lose another child!

I'm so very sorry for your loss. Do you think she might be more affected by the loss of her sister than perhaps people realise, maybe more than she realises? Are there any other things that have happened, not necessarily parenting issues but just life experiences and losses, that may have damaged her? This all has to come from somewhere and I'm not blaming you, just wondering what has caused this, because something has. Doesn't make her actions acceptable, of course, but knowing what the cause is is likely to help with finding a solution.