That sucks, brother.
I had that happened to me too. When my best friend at the time had his heart broken by some chick who was a mutual friend, I kicked her to the curb immediately, despite her trying to seduce me (and she was a hottie). I take friendship with a brother very seriously. Later, when I broke up with an ex, this best friend left me at a club one night to give her a lift home when she showed up with some cute girlfriends, and they were only barely flirting with him. He has no chance with her or her friends whatsoever, she only wanted to piss me off a little. I didn't even have to make the point to him. After that night he was no longer a friend.
Those so-called friends are easily blinded by pvssy. You DO NOT want them to watch your back, that's for sure.
You are young, you'll find better friends. I did.