I call shenanigans.

As a past bar musician...there is always a 50% chance your set is going to be delayed if there is a big game/local game on the TV. Games are big money makers...dj's and some musicians are not. If he did not know this, he's just starting out or he's oblivious.

Your friend is probably more upset that she didn't get to be the center of attention than because you left. And If I had to wager a guess, you are more angry that you spent the entire day trying to make her bday special and she wasn't very gracious. That and you don't understand how someone could spend that much time planning a bday night and not even look at a calendar or talk to the person who booked the gig. You probably have this nagging feeling that she knew all along the gig was going to be a late one and they were both playing dumb. But considering you described her as immature...odds are both her and her boyfriend are just self centered, socially oblivious morons.

So my advice is to just let it go. You now know what kind of person she is, so you can adjust your expectations for the friendship accordingly