What should I do about my friend?

Okay, so I have this friend. I think I'll call her Sally because I like that name. Okay, so one day at lunch I'm drawing a comic strip for my friend who asked me to. I drew it based upon an event that happened earlier that day while between cles. Only she and I were there at the time, so those were the only characters I drew. Another friend of mine said, "draw me in your comic!" and started to draw her own comic. I said no but she kept telling me to so I drew her in the last box saying "hey, why didn't you put me in this comic?" with an angry face. Apparently Sally didn't like that and so she scribbled out the box and even ripped it a little. When I got mad at her because it was for my friend, she said, "so what? That was mean.". Then she showed me her comic in which she made fun of a lot of people. She put me in it so that I looked really fat and i was saying that I liked someone who in real life I didn't like. So then, (and I'm sorry this is so long but it's a long story) I told one if my friends who is a guy that sally put him in the comic too and he was mad. After school, my friend, Sally, and the boy were all talking with me when Sally said "oh Billy (let's just say that's his name sorry if these names r crappy) said that you were taking about me behind my back.". So I told her that was because he was in the comic and he was mad at her anyways! Then she said "well that's not nice, and it's not what friends do.". I told her she wasn't acting like a friend either but she just got mad. I don't know What to because my friend doesn't want me to get in a fight because she would just be a bridge in the middle. Please help!