I do know how hard this is, I have been down that road....
There is no right or wrong answer here.
There is only the answer that YOU can know, and the one that works for you.
Somewhere nestled between right and left, there is a middle. A place where there is a balance of how you are and what is right for you, and what is wrong for you...
Each situation is different, and nobody else lays their head on your pillow at night.
Confronting her....
What does that look like for you ????
What does the outcome look like for you ?
How does that play into the person you were ?
And want to be now ?
What does it accomplish ?
For me, when I was faced with this, I chose to make it more about things that I would or would not allow in my life anymore.
For me, if she wanted to be with another man, I could not control that. And I refused to allow myself to continually dwell on that aspect of it. I was not going to attempt to control what she did or didn't do with her life. As screwed up as she was, he wasn't getting all of her anyway, he was getting the used up part that she was burning off. There wasn't anything deeply emotional that he was getting.
To be honest, in some small way, I am grateful that she had someone to talk to through that part. That she felt she had some support through a very rough time, for her.
What I DID do, was to work through my anger, and come from a rational state of mind. And what I told her was that I could not allow myself to live with a person that chose to lie continuously to me. And if that was her choice ( to lie to me and our family) , then she would have to do it without me. And as long as there was another man present in her life, I would not be.
What that did, was to let her "own" her decisions, and to take that weight off of my shoulders. Why should I carry HER secret with me ? Why should I cover HER actions with secrecy between us ?
This was something that I did between us. Not family, not friends or coworkers. This was between us....
Try to answer those questions for yourself...
Know the answers to those, and that will allow you to find that balance for yourself.
There is no right answer here, only the one that fits in with your changes and goals....
Use that anger that you have as a shield, not as a sword...
It will not serve you well with matters of the heart.