If I got into a such a bad fight with my friends that I decided on storming out of a concert I'd been been very excited to see, the first thing I would do is demand my keys back and tell them to find their own ride home! I sure wouldn't leave my car with them. And I think it'd be very unlikely that it would slip my mind that they had my keys -- in fact, that's the first thing that would come to mind when my friends were misbehaving. "I let you drive my car/gave everyone a lift up here, and you're being nasty to me? No way. Give me my keys."
Now, if the friends then refused to give me the keys (maybe because I was intoxicated?)... my anger would go supernova. But you know what? At that point, I'd definitely call other friends to complain/arrange for help in getting the keys back after the concert/vent my disbelief/whatever. (And the whole "nine" was not at the concert, right? So maybe, at that age, I'd also call to shore up support in case it turned into a group-splintering feud.) Either way, I would not simply walk off in merry unconcern, leaving my car with people who had just behaved badly toward me.
So... if she did have a fight with her friends, I think the possibilities for why she left the arena (without her keys, and perhaps after contacting...who?) potentially grow murkier and even more confusing.