Do I snitch? Please help

I don’t know what to do in a situation like this and I’ve been thinking if I should keep it or snitch.

A mutual friend of mine confessed to me something extremely insane which made me see them completely different and feel horrible for the other party. So said friend had told me they were messing with a dude that had a girlfriend already, I was completely appalled by that because I also know said dude, we used to be friends. I used to be friends with said dudes girlfriend too. Recently, a few days ago, my friend told me they had gotten to the physical stuff (if u catch my drift lol) and I was completely shocked and weirded out because prior before anything physical happened I told my friend to leave that guy alone because he’s weird and said friend wouldn’t be able to have him in the end cause he’d probably do the same to said friend, obviously said friend did not listen.

I cannot see said friend the same after what they told me because i do not agree nor do I want to be seen around people who homewreck and cheat. I’ve seen said dudes girlfriend at my lunch as well and I’ve been thinking about telling her to check his phone or telling her straight up that he’s cheating on her but at the same time I promised to keep this between me and said friend. What do I do?