You know, I am absolutely baffled by some of the responses on here!! Absolutely baffled.
There are people on here who are suggesting that the OP doesn't say anything to the partner because they are not married and don't have children and this might be the start of something beautiful..?!!!
If it were the partner posting on here saying she'd found out about the OW, people would be telling her it was fortunate she'd found out now before marriage and children were in place and it was easier to cut her losses and find someone who deserved her and that, in the early days of a relationship, they should be so loved up he wasn't even looking at anyone else and the sad truth is that he just clearly isn't that into her.
In fact, I'm pretty sure I read a thread that looks exactly like that earlier this morning!
It doesn't really matter what the OW's (or anyone else's) motivation is for telling. If it is the truth, the facts remain. It doesn't matter whether the OW is bitter, or vengeful or whether she is hoping he'll leave and she can shack up with him, or whether it was entirely innocent and she knew nothing, or whether it's a friend who got drunk and succumbed to a husband's advances... it doesn't matter.
I never, ever understand the advice to not say a word about it, because it is far preferable to know the truth and make a decision about your own life, than have others know stuff about it and take those choices away from you.
I do wonder in these types of threads whether a lot of the vitriol towards the OW is because betrayed wives are projecting the anger they couldn't direct at the OW in their own situation; or whether they are angry out of fear that there could be an OW at some point in their future; or whether it's just a bit cathartic. But there just doesn't seem to be any logic to it at all.
And FWIW, I have a male friend who is married. They have children. There is absolutely nothing to suggest he is in a relationship on his fb. No photos of his wife, no comments to/from her, she doesn't 'like' any of his posts. Absolutely nothing. If I didn't know differently, I would have absolutely no reason to assume he wasn't single.
OP, you did nothing other than trust a man who you thought was interested in you and could be trusted. You found out otherwise and have ended it. I, personally, think you should say something. If I were his partner, I would want to know. She may choose to stay with him. But that is her choice to make.
There are some very bitter women on here who are unable to give impartial advice or think clearly when it comes to infidelity.