First let's look at this for what it is: Fact - he kissed a girl while drunk at a party/bar. Fact - he apologized for it later. Fact - you are both new to the college and dating scene.
He obviously felt guilty enough about the incident to tell you about it - which is good. A real asshole would have never said a thing and you would have found out from a friend or a facebook post instead. Give this guy another chance.
The real issue here doesn't seem to be his possible drinking problem. Helloooo, how many times have you been a bit tipsy and looked around the room and thought to yourself, 'gee that guy/girl is hot, why don't I go over there and talk to him/her.' When most people drink (especially young people), our natural mating instinct surfaces and this kind of behavior can overcome any logic that you normally use to keep this stuff under control.
Sigmund Freud wrote many papers on the subconscious mind and how we repress desires and other fantasies because our culture/society/upbringing shuns them. While I'm not making excuses for this guy, I just wanted to put that in perspective. We all tend to drop our guard when we are drinking - to some degree.
I'm just saying that she should give him another chance. Should she worry about him going out without her? Maybe, but the tru test will be if he does it again. One thing that is not mentioned here is whether or not the girl in question started flirting with him or he approached her first. Again, not making excuses for him, but it does take two to dance