Strictly speaking, he's done wrong. But we are humans, and we all err. Sure I've done something stupid, not just once too (Gordon you're right on this
)
I just feels that everyone needs to be responsible for their own actions. At the same time, we all need to take care of ourselves in the presence or absense (esp absense) of our partners, if we love them. It's a kind of responsibility and commitment to them that says hey, even though we're not physcially together, you're still in my thoughts and I'll be keeping you in mind no matter what I do or who am I with.
So by allowing himself to be that drunk and to commit that mistake is already something that is killing him.(I'm guessing that's all) By keeping it quiet and pretending that nothing's happened in front of her sounds like another suicide.
I mean, sure, I agree that "What you don't know don't hurt" But if he is living with such guilt, I'm sure it will find its way into their relationship.
But if he can live with it and life goes on, then it's up to him to decide what will be the most appropriate decision. Not necessarily the best, but the most appropriate one.
Most important is to learn from this mistake and never to do it again!
I'd like to post this question too: "What if now she's the one who made that mistake. How different will things be?"