Advice on violence with teenage girls?

A few years ago I became best friends with a girl two years younger than me (she acted much older than she was, being raised by a single mother) after she began horseback riding at my barn (we do not go to the same school). When she got into her freshman year of high school, she started dating a 19 year old with a swastika tattooed on his arm and started smoking cigarettes and weed. We got into a huge fight last November and since then it鈥檚 been hell. I鈥檓 not trying to completely trash her, but to put everything in perspective, I take school seriously and have never smoked anything in my life. She smokes constantly, does not do well in school, and has been suspended from school several times for fighting with other girls, giving one a broken nose. Since November, we have gotten into a few verbal arguments at the barn and on facebook. She constantly threatens me but the barn owner wants nothing to do with it. I have thought about leaving but do not want to leave the horse I have trained and leased since he was a baby, and the owner also refuses to sell him to us. We recently got into another fight, and she has started threatening me again. If she hits me I would rather not hit her back, being two years older than her. My parents have said they would sue her mother if she ever hurt me, but I鈥檓 not crazy about that idea either (I don't want to look like even more of a stuck up spoiled rotten daddy's girl than I already am). Advice please?!!!Advice on violence with teenage girls?
let her go! she let this guy get to her! you dont need a young immature friend like her.