The only thing you really can do is talk to her and apologize. People say dumb things sometimes. If she's really your best friend, she'll forgive you. 7 years is a lot to give up over one argument. I fight with my best friend (she's been my bestie since we were in 1st grade, 8 or 9 years ago) a lot, too, but we're like sisters, so it's really normal for us.
Fights happen. Apologize, and be sincere about it.