You should probably tell him. If others know he'll find out eventually.
If I were him, I would want to know what you plan to do so that this never happens again. If you plan to quit drinking, then you will want to tell him that. If this was a one-time slip up and you don't think you need to quit, then come up with another way of making sure this doesn't happen...
Above all, the number one worst thing that drunk people do when apologizing is say "it wasn't really me" or "it wasn't really my fault" (I've been on both sides of this coin
). That sucks to hear because you feel like, all right, so this was just an act of the universe that could happen again at any time and I'm not allowed to blame you? You have to own up, take responsibility for the poor decisions that put you in that state, and tell him how you're going to prove to him that it will never happen again.
(On a side note... if you don't quit drinking, maybe don't party with that friend anymore? She certainly didn't do much in the way of looking out for you. Friends do NOT let their blacked out friends have any sort of sexual interactions. I literally tackled a friend to the ground once in college because she was blacked out drunk and belligerent and trying to make a really, really bad decision. We both got bruised but she was thankful the next day. Not that it's your friend's fault, but any time you're doing substances, you should be with people you trust to look out for you if things go bad and who you would do the same for).