I asked him if it really was the alcohol's doing, he said "maybe 50/50"
I asked him if he liked her, he said "i dunno"
I asked him if he felt this was a "*** for tat" and he responded with "maybe, .....I dunno"
The above sounds like the most 'sketchy' part. "I dunno"...It's either "yes" or "no". "I dunno" seems more like "Yes, but I don't want to outright say that to you."
There's always the possibility that nothing more comes from this.
There's also the possibility that he could string you along until this other girl (he clearly has some interest in) breaks off her relationship and he goes for that.
The thing that seems different is that she's always been a friend of his, he's confessed to liking her in that way before and now this has happened. I would personally struggle to see it as a one off thing that won't happen again. They have a history.
Yours was just a one night thing.
I've been in a somewhat similar situation where I was with my ex 6 years.
About 2 years in I got absolutely wasted one night and slept with another girl. I don't remember the event at all, I was literally
THAT wasted that I have no recollection of it happening. I didn't actually find out that I had slept with her until about 2 weeks later (it was at a friends house, so there were witnesses to it happening...Plus 'other' factors that made me know it was true).
Being wasted obviously isn't an excuse though. It just begs the question of whether I was that into the idea, or whether she talked me into something when I was in a state where I couldn't even make a decision of what type of Pokemon to use against a Rock type.
I told my then girlfriend the moment I found out. She asked if I felt anything, I said no. She asked if I wish I remembered doing it and I replied "well, yea. If I'm going to do something stupid I should at least remember it", blah blah blah. We argued, cried, etc etc. All in all, I was entirely truthful, she forgave me and nothing else like that happened again (and I controlled my drinking a bit more)
A few years later she did the same...But she did it with more guys and I'm skeptical she was actually drunk with most of it. Regardless, we took a short break but I overall forgave her.
About a year after that she went on holiday again (which is where one of her past 'cheats' happened). She came back, told me she had cheated on me a few more times and that she was now moving to that country because apparently, I was just getting strung along until everything was good to get with this other guy (she didn't say that but it seems obvious it was that reason to me).
I guess you can't really trust somebody once they've done something like this is what I'm getting at. They may never do it again but there's the very real chance they will. I never thought she would do it in the first place or do it again after it happened but, hey, I was proven wrong on that.