How do I deal with a person who "bullys"?

So there is this girl I know. She is my friends friend and we DO NOT get along. I try my hardest to ignore her, but now its getting serious.
She has threatened me, called me names over facebook, spread rumors that arent true about me, and my ex had sex with her when we were dating… It is now to the point where my friend is taking her side more then mine and the whole situation is a huge crappy mess. I dont know what to do about it anymore and I am at my witts ends putting up with it. I cannot handle it all.
Any suggestions on what to do about her bulleying and what to say to my friend??