How to get revenge on my "suppose to be" best friend?

ok, so i have this friend and i met her about a year ago,we instantley became best friends and now she knows some stuff about me noone else knows. i know stuff about her aswell though. but recentley she has been acting like a total ..... there is this new girl to our school and used to go to school with me at a different school about a year ago, she is new this year and has became bestfriends with my friend, but this girl isnt nice. she is rude and twisted. she is now trying to befriend me to get to my best friend.. so this question isnt about the twisted girl it is about my best friend. while the rude and twisted girl trys to become her best friend her personality has changed, she has become rude and abnoxois. well the other day she came to my house and wanted to get a ride to her hous eso she wanted to use my phone and call her suister to pick her up. her house is like 5 mins walk from mine and it was a nice day so i said no... i took the phones and locked myself in the bathroom. she got mad and stormed out my house... i was going to let her use the phone though but she became rude and B..... so i shut the door and went inside .. i was so mad. this was on friday and on saturday i tried to talk to her on facebook cuz she had my sweater and i want it back... then i give this really nice apollogy and she says she duznt care about the phone thing .. she called me a B**** and rude.. shes like im not going to lunch helpers with you and i said i didnt want to start anything and she said i already did... she has been like this for many months... i gave her alot of chances and she is still being rude. i want to tell her to stop and were not friends but i cant becuz she knows some secrets about me that i dont want getting out. What do i do? i need to tell her off but i cant and please dont say revenge isnt right or gett a principal involved because that wont work with her... she is my best friend but she needs to stop talking about me and decide and learn who her true friends are... PLEASE HELP!!!!! I NEED AN ANSWER BY THE MORNING!!!!!!!!! p.s. she said that she didnt copy my lunchbag when i had it first then she went out and bought it!!!! she is rude but is my bestt friend!!!!! and she said she waass mad at me for some reason that i dont even know what it is??? helpp!!!