I think one of the bigger problems here is your drinking. are you looking at that? that it could change your character enough to deceive your best friend multiple times. is it time that you looked at your drinking and did something about that as well?

is the girl willing to sit down with you and tell him? that may be the best way to go. otherwise, just tear the bandaid off. let him know that you have been dishonest with him and let him know what has been going on. there is no easy way around this. good luck.