Quite honestly aside from the drinking issue and the obvious neglect of his girlfriend and clear priorities being messed up, I surmise from your description that there is a lot more to it than either of them are letting on. I'm sorry but at 2 am going to a female's apartment to continue drinking is very rarely an innocent, friendly meeting, I dont' care how much one denies it, it's just not typically the case. The fact that someone is saying that he was too far gone, seems to be a bold faced lie too because in that awful intoxicated state you do not invite someone over to "chat" some more. you're past chatting stage and I don't know about others but in a highly intoxicated state I'm not very conversant anymore just kind of silly, stupid and incoherent.

Even if I'm wrong in my assumption here, hes stupid and arrogant for forgoing a night with you to be with another person to drink, regardless of if it were the "boys" or another woman.