was the other guy drunk too, or did he take advantage of the fact that she was drunk? I think it should be up to the girl to tell your friend. If you really value both of them as friends, and want to remain friends with both if the relationship ends, than you should stay as far away from all this as you can. If he's your best friend and shes just some girl you've known for a while but aren't great friends with than I guess you could tell him. I still think it should be up to her to do it though because shes the one who fucked up.