Is it my responsibility to say anything to him? Should I give my friend an ultimatum or just forget about the whole thing? That's her intention. Forget. Put it in the past. They've been together for close to two years.

Why I feel conflicted about not telling him is that the three of us hang out all the time, and he is a good friend of mine. He's a really good, HV, kind hearted person.

Anyways, my friend. She kissed another dude, that she has history with, at an event she was at some weeks ago. She called me half drunk after it had happened and asked me what to do. I told her I'd tell him if I were her. The guilt and shame alone would have eaten me alive. I urged her to do the right thing, to which she said nothing good would come from telling him. It'd only hurt him.

Well duh.

The few times I've mentioned it afterwards, she's quickly and sternly shut me down, telling me not to tell him, and changing the subject.

If it's at all relevant, she has confessed to me that he has a low libido. I told her to go to couples therapy for that, and to not cheat.

Edited to add: we're late to mid thirties.