It doesn't really sound like that person is so much on the side... Dude, you're young, expect shit like this for the next couple years. You should be out mixing it up with as many hotties as possible anyway.
As far as drinking, you're just using this as an excuse to feel sorry for yourself and give yourself permission to do something you know is wrong -- you know better. Really, it's your life though. Do you want to be a 16 (or is it 17) year-old drunk? Or do you want to be the guy who works out, has respect for himself, confidence, and knows that if one girl doesn't work out, there are many others out there for him? Your choice bro.