If your best friend was cheating on their partner would you tell them? This can be a messy situation no matter what side of the coin you are on. I've been in both and I can tell you its never fun. Personally I try to stay out of other people's relationships as much as humanly possible, however I believe that in order for relationships to be successful you have to always tell the truth. Lying can be worse than cheating and keeping the truth from people can be just as bad.
As stated earlier I've been on both sides of this coin. A former friend of mine was cheating on his partner and I just didn't like being involved in that mess. He would bring both guys around and they looked alike so we were mixing their names up. I spoke to my friend about it and he got in his feelings so I finally just said don't bring either one of them around me. If you want to cheat you can do that on your own time and in your own space.
I've also been the one that was cheated on and my ex's boyfriend didn't say shit to me the entire time. What pissed me off most about it was I considered us to be close enough friends that he would have at least given me a heads up, until I realized birds of a feather flock together and he was a cheater too (he was trying to fuck me on the side) There is no honor among theives ya know.
So what would you do?