to want to tell the OW about the affair?

[Karol2]

Friend had been having a relationship with a guy who she thought was separated but turns out is married.

I am so cross on both hers and the wife's behalf that I want to track down the wife and let her know even though it's not really my place.

AIBU?

If I were the wife I would want to know. If there were evidence that my partner, husband, whatever, were cheating then they aren't the person I thought they were and they clearly wouldn't be respecting me if they had an affair.

I would want to know so that I could make decisions when fully informed. Because I'd rather live alone than with someone who thought they could cheat.

I’d want to know if it were my DH. I’d be more likely to believe it from the OW herself though not a friend of.

Support your friend.
She did wrong but finished it as soon as she knew. She will be grieving the relationship that she thought she had as well as being angry at herself and him.
Be there for your friend.

So what happens if the OP tells the wife and then the wife starts contacting the friend and causing problems?

How has the OP helped her friend by doing this?

Littlechocola I don't agree the friend did wrong. She, probably naively, thought it was a regular relationship with a separated person and ended it as soon as she knew he wasn't separated.

These days I'm cynical enought that I'd be researching online, etc, if any potential date said to me they were separated.

Least said soonest mended

It may well be better for the wife not to know the details. It's not always better to know everything. How could your friend have had a serious relationship without having any idea the man was still with someone else? Sounds very complicated and I definitely would not get involved.

I have been the wife,IMO you should tell her though not aggressively or through social media,it's humiliating and heartbreaking to find out your husband is cheating on you and other people knew but kept their mouths shut,I've spent a long time trying to get over the humiliation of being played for a fool while everything went on behind my back,this could be something the wife has suspected but not had confirmed

IPretty horrible thing to say.
What is up with some people. She found out something horrible which has destroyed her yet you decide she’s the Bitch in this!
Amazing*

You know, I didn't mean it that way at all. I'm just going on the threads on Mumsnet where when an OW gives the history of the relationship there is usually this sort of happening early on.