Also, while it is good advice to tell the other spouse if you can find her, it's not a solution to your problem. Your problem is not the AP and his wife, yes telling her might cool AP's jets, but you still have the problem of your wife. YOUR WIFE is the problem. She is very disrespectful of you and very uncaring of your feelings, your marriage and what happens with your son. That's only on HER, not on his wife, not on her parents....on her and what she has been willing and what she wants to continue to do. You have to make your own position absolutely rock solid and perfectly clear because she will try to take advantage of any lee way or any weakness that you show. IMO, you need to show her an action directly from yourself, and to me, that is going for a divorce. I know you don't want it, but....do you really even want to be with someone who treats you like this? Who has to THINK about whether shew ants to be with you or some married gym rat - your wife has no respect for marriage - yours or HIS. Is this what you want? Is this relationship acceptable to you? Only you can answer that or how much or how long you are willing to put up with this attitude and this treatment. YOU SHOULD GET MAD.
I think the situation depends on how YOU handle it at this point and that will set the tone for everything going forward. DO not be manipulated by other people or get distracted. This is about YOU and your values, feelings, ideas about marriage,etc. You do not have to put up with this shit for ONE DAY. Now, yes, she could say, okay, let's get divorced - if she says that.....you know where she really DOES stand and what you mean to her. That's a clear answer. If she tries to say she'll stop it....well, I'd continue with the divorce until I felt convinced of that because....she'll probably try to take it deeper underground. You can always stop a divorce filing up till the end so filing is not the same as BEING divorced, it's a clear intention though, and it's real action.
So yes, you can contact the other spouse if you can, but she may not believe you or want to hear it anyway, or she may just be like, yeah, this is what he's like, he's no good. Or she may lay the hammer down, who knows. But it's not going to change the essential problem which is that your wife wants to cheat on you and she can find someone else. You're not Number One to her and that is what must be addressed and fixed, if it can be.
DO NOT TAKE SHIT OFF ANYBODY. PEOPLE WILL ALWAYS GIVE YOU AS MUCH SHIT AS YOU ARE WILLING TO TAKE.