Tween girls

So what to do about the evilness of some tween girls? Right now, my daughter's best friend has decided that they're not friends anymore. And she has started whispering and pointing at my daughter. And having other friends tell her "Susie (not her real name) hates your guts". So nice.

My daughter has always been pretty secure in who she is. She gets a lot of kudos for how good she is in gymnastics, how active she is, etc. But i see a change creeping in now that she's a tween. It's very subtle, but she's starting to become more shy and less outgoing. Maybe it's just the recent events at school with her best friend, but I feel like clutching on to her and screaming "nooooooo!" as she fades into sullen teenager-ness.

Her light is fading, and it scares me. I'd love to hear from others who have either been a tween girl or who have daughters who has gone through this.