I see several reasons for concern. One, she had been arrested for public intoxication before. That does not happen unless you are really messed up and unable to control your self. Especially in a college town. So he had good reason to be concerned about her being out drinking without him. He would know that she sometimes drinks too much and is potentially 'out of control.'
Two, she had plans to come and be with him. So he knows she is out drinking and then she stops answering her cell and is ignoring the plans they made.
As her boyfriend, that should make him feel worried about her safety, if at 3 am she does not answer his calls, after making plans to meet up. Who isn't worried about a pretty young drunk co-ed out in the middle of the night ?
And underneath the concern there would likely be some jealousy. His gf is very pretty. So he has thoughts of his petite, blonde gf, smashed, wandering around Kilroy's, hanging out with whomever. Why wouldn't he be jealous?
They dated for 2 years, is that right? Sounds kind of serious.