It depends who is doing the cheating/being cheated on.
If it was my BFs husband or partner cheating, yes would totally tell her.
If I didn't know the couple that well or they were just acquaintances, I'd mind my own business.
If it was my friend doing the cheating - and this has happened - I'd strongly encourage her to think about what she's doing, but I wouldn't wade in and tell her husband - that's down to her.
When she was having an affair I found out by accident. Her DH was horrible, frankly, and I didn't blame her, but I suggested that she would be better off leaving than cheating. As it turned out, MY husband decided the horrible DH should know and engineered a situation where he had to confront her.
It was awful, but a good 10 years ago and everything has settled down now (BF now remarried) - but the horrible DH is still an acquaintance and he has never really forgiven me for not telling him earlier myself.
All I can say is, damned if you do, damned if you don't...but nobody will thank you. If I had that time again, I would have pretended I never saw a thing. Too complicated.