How can some people just throw away great friendships like that?

[Erica]

My ex best friend (let's call her Beth) who we have been friends for 10 years has changed a lot. Since this summer, Beth has been hanging with these groups of girls right?. Since my boyfriend and I started dating in October, Beth has been flirting with him constantly. She even did that RIGHT ininfront of me! My boyfriend and I confronted her about that. I told her to quit flirting with my boyfriend. These girls that Beth hang out with are superficial and nasty. My friends and I have been telling her that them girls are no good and saying untrue things about us. We tried, but Beth won't listen and just took their side and she KNOWS that the rumors are not true. Even she started saying nasty stuff about me and my friends behind our back. We even had a fight about it she she called me a "bitch" and "skank". Now she wants nothing to do with us. This really breaks my heart. After 10 years of friendship, we done everything for her! Everything! We had her back and it hurts now. :'(