Throwaway account, just in case. Me (male) and ex-wife (female) and I recently ended our relationship and marriage. When going through my things, I found conversations on a shared device that proved to me she was cheating on me and in a sexual relationship with another man for quite a bit of time while we were married (over 6 months). In these conversations, I also discovered that the man she was cheating on me with also has a wife, who does not know about their sexual encounters.

I do not personally know his wife, but I can't help but feel like it is the moral thing to try to figure out a way to contact her and let her know this information. Most of my friends have advised me to just move on and forget about it, but I feel like it is the right thing to do because I am not really seeking out revenge, but because if I was in his wife's situation, I would want to know. Also, I definitely feel like I would be ok with whatever she decides to do with this information, including staying with him - I feel like my moral obligation begins and ends with making sure she knows.

The main potential downsides is that this course of action does come with its dangers - for instance, if the 'other man' decides to come after me for ruining his life, etc.