Personally, I would confront, but not until I knew FOR SURE. I know this is contrary to the DB stance of letting it play out, but I believe women lose respect for a man who allows them eat cake. So, especially if my W knew that I knew, I'd confront.
If you let it play out, you have to pretend you don't know. Can you do this? Will it eat you up inside?
I would say something like, Look, I know about OM. I'm not interested in any details or explanations, I just need you to drop him for our relationship to continue. Then sit back and leave the ball in her court while being dim. No benefits of being your H. No taking your son so she can do an extended holiday with OM, Etc.
Best wishes to you rky, stay strong my friend.
M: A really long time. Crisis: 5 years. She's still worth it.
Life is never made unbearable by circumstances, but only by lack of meaning and purpose. -Viktor Frankl
