There may very well be nothing you can do about it. In my case, I knew about my wife's EA, but the OM's wife didn't. For two years I didn't tell the OM wife because 1) my wife asked me not to, and 2) I was afraid it would push my wife to him if his wife knew and left. Instead I suffered in silence, battling on my own each day. I asked my wife not to contact him, and she assured me she wasn't and wouldn't. Those words were lies. Meanwhile, the OM had no such pressure, he could call my wife with impunity. When I finally dragged the affair into the light of day by letting the OMs wife know it took all of the magic away. All of the Romeo and Juliette stolen moments on the phone. Suddenly he wasn't "Prince Charming", he was a guy that had a wife and two children that he was cheating on. It was out in the open. For me, it worked.
I did everything in my power to speed the end of their affair. Sure, you could sit and wait, and true, you have no control whether you expose it or not- but it speeds up the end by taking the most attractive part away- the fantasy.
You must make your own choice- its your life. If his relationship is already that skewed up, I seriously doubt that letting his exW know would do anything at all but perhaps make a bad situation worse. Additionally, I do not advocate telling the world about the affair- especially family. If you do R, they will never forget and it will strain you M. Again, just my opinion.
