Friend help? I Don't Know What To Say?

ok so i have been best friends with this girl, lets call her ella, anyways i have been best friends with her for about 3 years, we had somegood times, but she changed, she got all violent with me and everyone and she always dyes her hair whacky colors and she dresses really wierd on purpose, and she talks back to the teacher and is 1 in cl suspensions away from being suspended, i mean she always gets made fun of and when i met her i felt really bad for her but know i know why, because shes sort of indifferent, and she didnt let me go on my own compute when she slpt over she played on it all nightand on my facebook (at 1am when i was sleeping and she was on my computer) i saw my status and it said "i hate ella" and i didnt write it! and she denies she did it, and she said a whole bunch of mean things about my other friend, i was mad at her at the time so when my other friend, lets call her "mya" i toldmya eveything ally said and she got mad, so at school ally got mad and she called mya a bunch of rude names and said she could never trust me, so then she called me and started dissing me, so all i said was "ok" so then te next day she said she was sorry, i dont know what to do, she not a good friend at all, I DONT KNOW WHAT TO DO