Ok heres my problem...

One of my friends wrote an email to her friend asking if she would be more then friends with her (they are both girls) so she said no and she printed the email out and shew loads of people at my school and asked everyone not 2 say anything. When my best friend found out she went on one of the sites that she knows the girl goes on and made up an acount pretended she wos another lesbian girl and started talking dirty yo her to she wot she says, so the girl spoke dirty back, my friend printed out the conversation and again shew everyone. My friend has been speaking 2 her other "friend" thinking shes someone else for ages because wot they are planning on doing is going along with it for as long as they can and then gonna ask her to meet her and then they are all gonna laugh at her and gonna stick copies of the email and the conversation all on the school walls. Now ive been trying to stay out of it all but my best mate and the others who are involved are trying to get me to do it with them but i feel so sorry for the girl, they are spos to be her friends! so i feel really guilty but also dont want to say anything cos my other friends will get mad at me for saying anything. What should i do? Hope it all amde sense BTW.