How do I break off this friendship?

[Tony]

Okay. This guy I was once friends with started hating me. He's a senior and I'm a sophomore at my high school. His reasons were pointless, that's when I realized how pathetic he was. But my best friend continued being friends with him even after he would talk shit about me on Twitter for everyone to see. I let that slide, it bothered me that my own best friend since KINDER would still be friends with someone who is doing this to me but I left it alone. Finally this senior and all his stupid friends kept telling me I was gonna get beat up. So this senior cowardly hits me from behind one day at lunch and before I could fight back the teachers were already at my side breaking everything off. I asked my "best friend" if she was still gonna be friends with him. She said "No, he's a pussy for doing that. I don't like him anymore." Well they've still been conversing on twitter and I just saw them today walking in the hall together and she completely ignored me. I was beyond done when that happened. I feel so betrayed after telling her numerous of times to break off their friendship because if she was in my position I would've done that. I saw her today and ignored her and her friends have been telling me that she's sad because she thinks I'm mad. I'm not mad, I'm done with her. How do I tell her I'm done? I want to sound confident, no mercy but not rude or mean. How do I tell her?