I recently lost one of my best friend due to lack of communication, and lack of forgiveness. On both our parts, mind you. I made excuses for myself, and her behavior, and never looked things that bothered me right in the eye and addressed them, because I thought that being in a pseudo relationship would be better than making her upset or angry.
That being said, you have what I like to call, a break up. You can try and remain a distant observer in their life, their social media presence and their day to day presence, until it all becomes too much. Just like when your heart breaks in a romantic relationship, it breaks when you lose a close friend.
You lose other friends because of certain things, or you split up certain friends. But really, I truly believe if someone chooses a side against you, or leaves you or breaks your heart, they really never considered you important in the first place. Now, you can address these people, look them in the eye and ask why. Why are they abandoning you? Why do they side with her? Why do they think her side is anymore justified than yours?
If they were true friends, they would talk to you, voice their problems, their concerns. If not, then I am sorry to say, but get the hell out of Dodge. AKA, move forward. If there is anything I have learned since conquering my social anxieties, it’s that there is a person who wants to get to know you right around the corner, and you should be just as enthusiastic to get to know them.
Good luck, and remember that who she is, what she does, and who is now her friend is of no concern of yours. Identify the essential people in your life and eliminate the negative. Either you fight for your friends or you don’t.