I had a similar thing happen with friends of ours - not the same situation, but I found out a good friend's husband was cheating on her. I walked into a restaurant while taking a client to lunch, saw him with a female other than my friend......he didn't see me but I saw more than I cared to.
I struggled with what to do - I didn't dare ask other mutual friends, for fear I'd be spreading around something so hurtful (my husband's advice was to either go to her or stay silent and he was right) - imagining how I'd feel if others knew something like this and didn't tell me. My friend was someone who was very up front, very direct and was appreciative when others were the same. I waited a couple of weeks and couldn't stand it anymore - I had been avoiding her and couldn't even imagine going out to dinner with the two of them as we occasionally did. My thinking was "our relationship has already been strained by what I saw, so what more harm could be done?" I called her, met her for coffee one morning and was very shocked to find that she already knew about it! It wasn't the first time he was unfaithful to her, either! She stayed with him because he made major money - she made decent money, too, but a separation would definitely have changed her lifestyle.
I am almost ashamed to say this, but knowing she knew and stayed with him after several affairs changed me and how I felt about her. And it goes without saying that I felt her husband was a
tit! We aren't close like we used to be, we don't go out with them any longer, but if we bump into them somewhere we're all very friendly and pretend we're going to "get together soon". The relationship we had has sort of drifted off and the pandemic only made that worse.
My advice is to take your own consideration here - protect yourself and your husband, which is why I liked
@Le Baiseur's advice!