Probably going against the grain here, but he may not know. I had no idea my wife had an AP for two years. Stumbled on an email completely by accident that led me down a rabbit hole and I finally saw what now seems obvious. We used to talk about how trusting our relationship was that it was great we didn't snoop on each other (stupid, stupid me). One day her phone suddenly had a passcode and it still never clicked. I thought the worst possibility was her texting with her girlfriend and bitching about their husbands like I knew they did (stupid, stupid me). She deleted everything but I'm a techie guy and was able to recover all their texts, etc. She was devastated when she learned I read them. She had lied a lot to minimize it and the texts showed that.
She works a lot and is home every night and she drinks too much. I never worried about an affair because she was passed out on the couch every night by 10:30 or 11:00 (incidentally, this is why I thought we had a mostly dead bedroom). When would she have one? Well, turns out she worked a lot the nights before her meetups with douchebag (every two to three weeks) so she could leave early the next afternoon and see him. Then she'd come home, brush "him" off her teeth, kiss our kid and have dinner. Now I see when her drinking suddenly got so much worse corresponds almost exactly with the time of the affair.
She begged me not to leave and divorce her. Said it was just sex and when she pictured her future it was always with me but I wasn't paying attention to her blah, blah. That's true, we had gotten pretty disconnected. Full disclosure, I had an affair very early in our marriage that she always suspected and I always denied and I think that contributed. I've admitted it now. We're in counseling, almost three months out from discovery. We talk sometimes for hours now, like we did when we met. Our sex life has become volcanic again, which is great but she sometimes cries after and says things like, "I'm so sorry. This is all I wanted but I thought it was gone forever." over and over. I don't know if we'll make it but she's done everything I've asked (counseling, STD testing, a few other things) and swears she's 100% all in with our marriage now. The aftermath has been brutal.
That's our story. It's obvious to me in hindsight but I never suspected. Her husband may be the same. Also, remember it's a very dangerous game your playing. I didn't know this guy, he was an ex BF from years ago. I've cooled down a lot now and see the blame lies with my wife and nowhere else. I found a rage in myself the first couple days I didn't know was there and never wanted to. I don't know any of you or your situations but If he had been a neighbor or "friend" who came to my house and knew me...wow. I owned a gun and if I'd known where to find him those first couple days I really don't know what might have happened. It scared me so much I've actually sold my gun now. Make sure you know who you're fucking with. I took it very personally, as something he did to me at first, and I've never even met him.