Bless you both!
I agree with Heather in a lot of things. But I would add that, being in high school, she should talk to the school counselor. In seeking an authority figure in the high school herself, she can learn a bit about choosing options for herself when confronted with behavior of others (which is not within her control).
Working on letting my teen have more control in his own life (and responsibility) is something I'm working on, though it's not easy! BTDT!
As for the parents of the "used to be friend", consider that they have been told a story quite the opposite. I bet it sounds very much, to them, as your daughter's side of the story. Being assertive without being aggressive or angry might be the best approach.
Good luck! You two are already doing wonderfully!