Ex-Friend Is Going Insane?

[Dressage61]

So I used to be friends with a girl who's two years younger than me (I'm a freshman, obviously the maturity level is very different) and well I started doing dressage after riding horses western at a not so nice barn that she leases at. She started leasing a horse and since then has been insane. She would hang with a bad group, started talking crap about me, and finally, I called her and cried about how I didn't want our friendship to end and she sounded like she didn't give a damn. Finally, we broke it off. Her Facebook was logged in on my phone and I was being nosy (I believe strongly in privacy but my anger drove me to look through her messages) and found her talking bad about me. Making lies up. She has two Instagram accounts and blocked me on one. Her active one, she has been using to show off her new friends. I still visit the horses and work at the barn she rides at and would like to return to work after three months. What can I possibly do?