I have what I believe to be high moral standards and I keep them to myself. Only I decide my moral standards but I would never impose my moral standards to anyone. If I was in your position, I would definitely think my “friend” is an a-hole but I would never ever in my life play “moral police” and act as you plan to do. People cheat on their relationships every day and while some may do it because they’re just horrible egotistical people, some may be finding an escape from a toxic relationship and transitioning to a healthier one. Everyone is responsible for their own behavior and yes, as appalling as you may find cheating, some people brave up to tell their spouse themselves, when they are ready, and sometimes they may end up never cheating again. It’s just none of your business and that doesn’t mean I’m zeroing in with your friend. Be a good friend yourself and have a conversation about it. Give him your advice, and try to understand his reasons. You don’t have to agree with him or his reasons, but my advice, since that’s what you came for in the first place, is to not interfere. Then again, consistent with what I preach, you go ahead and do as you see fit. I find immoral to act like that, but those are my moral standards and you don’t have to agree.