Geez… if that’s legit, and I hate to give this advice, say nothing. If you say something, you’ll absolutely be ruining his relationship, your friendship, etc. It MIGHT come out in the future, but you could just as easily say you don’t remember anything either - you were drinking heavily too. I hate giving really unethical advice but this honestly sounds like one of those rare situations where letting sleeping dogs lie might be the best thing for everyone.