His actions from that night on would be a big indicator in my eyes. Im not saying there is any excuse for cheating. Believe me, my STBXAH did his fair share and tried to blame it on his drinking and me and everything under the sun besides him just being a scumbag. But...he always lied and tried to hide it and if I found out and confronted him, he deflected the reason onto me.
IF the man in question continues to SHOW remorse and works to rebuild trust, if he has no issue with having std testing and attending counseling, if he shows that he understands that what he did was an issue...then it seems that it was a terrible mistake and not the start of a pattern....just an opinion though..
It sounds like his drinking is definitely a problem. Hard to say if he is an A or not, but regardless, his drinking is causing concerns and issues for him and the people around him and should be addressed.
I know from working with addicts and alcoholics every day that the idea that someones actions and words when they are drunk being what is already there sober but maybe not coming out...not always the case. I have seen people who have so much remorse and guilt over their actions while under the influence that they have taken their own lives. It just depends on the person.
My big question with this scenario....what is the underlying cause of him drinking until he blacks out? Is there a mental health issue? Past trauma he is trying to numb? Does he have addiction in his family? A mentally healthy person will not drink to that extent for fun....