Ages 10 and 11 were pretty hard for my DD. I think part of the meaness is related to the incredible changes girls are starting to go through at that time. Girls seem to mature physically at an earlier age, but emotionally they are still young. They are short tempered with each other and overly sensitive. A bad combination, for sure.
Mostly you have to give them time to work it out, but there was one time that I took matters into my own hands...
At 10, my DD had a "best" friend who was manipulative and domineering. They had another friend who moved away. That friend came back to visit for the school carnival, a huge event where it is wall-to-wall people, much like the Magic Kingdom was the week after Christmas

Those two decided it would be funny to ditch my DD. She found me in the gym, put her arms around my waist and buried her head in my chest, tears pouring down her face, because she'd been left alone. I got that feeling from the top of my head to the bottom of my toes that mothers get when their kids are hurt.
Eventually I saw the two little brats and I confronted them. I was stern and my message was simple. You know you should treat others the way you would want to be treated. Would you like to have been left alone at the carnival? Then why would you do that to Elsie? DD was upset with me for saying something and it put the nail in the coffin of their friendship. However, being associated with that girl had limited her social interactions with other students (she was new to the school district). Once she wasn't friends with the brat, she made some wonderful friendships that she still has today.
While the teenage years had their challenges, as far as the friendship thing went, that seemed to get easier the older she got. There's still drama, plenty over the Winter Break, but she seems to be able to understand what's going on and ignore it, or deal with it, depending upon the situation.
Good luck to your daughter; here's hoping for a better day!