My friend is a complete liar! Help please!?

Alright, long story short. There is this girl, who i thought was friend, and she is such a liar! She manipulates people! I found this out because, well, i snooped. We had been REALLY close, up until last month, when she met one of my other friends, started talking, and became friends. Now, normally i wouldnt have a problem, but now they both would ignore me, and they had inside jokes and secrets that they wouldnt tell me. They told eacother inside jokes that i had shared that was supposed to be just between me and one of them, and well, i became the third wheel. So, upset, i began to find out information about her, and this way, i found out that she was a complete liar, who uses everyone and lies to them. But the problem is, if i tell any of my friends, they will never believe me, cause she has manipulated them. The thing is, this girl is sneaky. She is always kind, and sweet, and gentle..... so no one will suspect her... but i know that she is a maipulating liar. So, guys, please help me. I dont know what to do! She says she hates all these people, then goes to them and tells them secrets, tells them theyre so sweet, and beautiful, and goes out shopping with them. It makes me sick! Please adivice? What do i do with her? Its summer vacay, and im leaving next week for the whole summer.... im worried to what she will do. Please help! Sorry this is so long, and thanks.