About 10 years ago my wife and I had become more and more social with one of our neighbor couples who lived on our street. The wife was an engineer at a local company and on a scale of 1-10 I'd call her an 18, scary smart, charming, and an absolute knockout. He's a smart guy as well, had his own business and was (is) very, very well off. She had to leave the country on a European project that lasted about a year and a half, coming home once every 2-3 months or so, and he would go visit her about once a month. When she was gone he cheated on her left and right with the hired help from a local bar. Nonstop. He told me story after story about his adventures and all i could do was tell him "Mike, why are you doing this? You really need to stop. You guys have kids." Didnt matter to him one bit. I knew where he was hanging out after hours and it was a well known dive full of middle aged desperados, and has since been shut down for.... illegal short term relationships, involving money. His rationale was "Well she's over there probably doing the same thing to me".
I really wanted to let his wife know what was going on, and so did my wife as she was more friends with her than I was, but there is no good way to do that. Once she returned home he stopped his nonsense but ironically told me he found she had been texting a coworker off and on and he went ballistic and showed up at her work and threatened the guy. He showed me pictures of the texts, I really didnt see much of anything, the conversations looked pretty platonic and like fairly innocent chit-chat to me. I pretty much concluded he was a raging narcissist, and one of my mutual friends with him chuckled at me for not coming to this conclusion a long time ago. We are still acquaintances but I keep this friendship on a very shallow level and we dont get together much at all anymore.
My advice.... plead ignorance and do not get involved. You'll lose the cheater for sure, and the spouse victim will only blame you for not telling them sooner, or for telling them at all.