Personally I'd keep my nose out of it, as even if you know (s)he's having an affair and even if your friend is all sweetness and light to you, you don't know how it is for their partner and sometimes you can love someone but they can be an unfulfilling partner in some way, yet you don't want to hurt them so have a careful affair. The person having the affair needn't necessarily be the bad person, it might just be their way of dealing with things.
I say this because I remember a woman when I was growing up married to a lovely man who was having an affair with someone or other, they were very happy and I remember thinking at the time that, but some clever "friend" of hers told her he was seeing someone else and she never approached him about it, just killed herself. Her friend was unaware that she was very ill with depression and her husband was putting up with a lot behind closed doors, so I'd always personally keep my nose out, nor would I wish anyone to tell me if they knew my wife was seeing someone else.I think a couple's private life should be exactly that, private to them.