The only person to blame here is the cheater.
He may have done a morally dubious thing but it's likely he will have to live with his guilt as well as looking over his shoulder worried people will spill the beans.
The OP isn't to blame because it's not her life she would be effecting. Ideally, the cheater would own up and let his future wife decide for herself. But we often forget as intelligent as we are, we are still animals. Sometimes we act despicably. Circumstances, intoxication, environment all play a factor.
This isn't to say it's OK, but it is what it is. Pretending we all have the moral high ground is disingenuous. Perhaps this man will decide he will no longer consume large amounts of alcohol when not at home as he doesn't trust himself.
The OP feels it's out of character for him so it doesn't seem like he went out seeking infidelity. But neither does it mean he will not nor should not be judged. People will judge him and that's completely on his back only.
I don't know what I'd do in this situation as it would depend so much on many factors. But I know I'd judge him and I'd also be there if she needed someone to talk to.
I once at 17 got called up by a friend who was on his stag and also one of his friends saying 'if X asks/calls we have not been to a strip club'. Then proceeded to tell me they were just about to go in. Why they needed to tell me/ask I don't know as if she had called I'd just have said I have no idea, which would've been true.
She did call and I said that her husband and said friend had asked me to say they hadn't gone to one but I said I have no idea if they did or were just covering their backs.
Both the guys were shitty with me but no fucks were given. Do something morally ambiguous on your own back but don't expect me to collude in it.
They divorced 3 years later and I still speak to both of them.