Patient, I read (skimmed) your other posts and saw nothing about drinking. That puts a different spin on things. I'm not surprised at her family life. The picture all begins to make sense now, doesn't it.
Are you in a program?
About her interactions with the other man, there are things we do in life that we know we shouldn't but we do them because they make us feel better. Many times they aren't good for us and we wish we could stop. We beseech all that's holy, bargain with ourselves and lie to ourselves "Only one more time can't hurt."
Can you have empathy for where she is right now?
I usually don't comment too much about affairs because that's not my sitch but I did have an EA years ago. It was all about the fantasy and things that my H wasn't doing for me at the time.
Only you know where your line is. But don't set boundaries or give ultimatums for the purpose of changing her behavior. That's not what boundaries are for.
Is she in IC or a 12-Step group?
Me 57/H 58 M36 S 2.5yrs R 12/13
Let me give up the need to know why things happen as they do. I will never know and constant wondering is constant suffering. Caroline Myss