Call for back-up
All in all, good job! Many people would have turned a blind eye. Sounds like you averted some serious harm, and it worked out well.
That said, what were the options to notify someone? If I had a cell phone, I probably would have called it in, hopefully to the Campus Cops' direct line. At least find a passerby, if possible: Vector in the back up, contain the situation, catch the guy (he's going to do it again).
Of course, I don't own a cellphone, and I wouldn't have left the girl either...so you go for it. Just know you got lucky.
I wouldn't advise carrying illegally, even a knife, especially if you're drinking.