You just learned a very important life lesson. This girl was never your best friend. She is two faced and you will now be more careful about who you let into your circle of friendship. Better to have a lot of aquaintances your friendly with than one good friend who turns out to be not so good.
By the way, I think she likes your crush and so therefore is out to make you look stupid.
There are a few rules to follow when something like this happens.
1. Don't stoop to their level. Walk away rather than let them ridicule you. They will only look rude and stand out even more prominantly if you act like a lady and don't take the bait.
If your crush was worth your feelings in the first place, he will not fall for the stupidity and will see what they are doing.
2. Be cheerful and laugh and don't let them see it is getting to you.
3. Don't associate with the best friend as she was never your best friend anyway. Don't tell anyone what you think about her, as she will be trying to see if she is getting to you.
4. Do something cool like throw a party or get together for your other friends, they are followers and they will realize you are nice and not believe her. It will show them that life goes on and you are not affected.
Basically the best revenge is letting them know that you are not affected and you will only look good to your crush. If he isn't impressed and prefers to hang out with your ex good friend, then find a new crush. You can't change what you can't change. Be yourself and be proud.
