R., This is a tough situation to be in. I have 2 wonderful daughters (9 yrs and 12 yrs) and this has happened to both of them. I think it is a right of passage!!!! It is currently happening to my 9 yr old... so I know how tough it feels to want to rescue your child. I would like to share my daughters' experiences with you. Maybe they will help you.
When this happened to my oldest, she was also about 10 yrs old at the time. The girl my daughter was having issues with was one of her good friends...something just went wrong between them. I knew the other girl's family and they were a good family so I decided not to address her mother at this point...I wanted the kids to work it out and learn a lesson. Instead, I spoke to my daughter daily and asked the teacher her input and to make sure things did not get physical or too mean. The teacher said it is a common thing for girls to go through this at their ages. Then I used this terrible time to teach my daughter how terrific she is and what makes a good friend, how to choose friends that are good for her..to be selective, what the difference is between friends and acquaintances, and how to deal with difficult people in school or when older in life in a work place. She applied this and did not take the girl's actions personal. She learned to avoid her and count on her true friends. Things gradually got better within about a month or two. A few years later, my daughter is currently best friends with her sister. She often hangs out with both and are all wonderful friends. Since there never was any tension with her mother, we sometimes do things with the kids with no problems or tensions. So hang in there...you never know.
Now, my youngest has a different story. She was best friends with a girl about 2 yrs ago. Now they are at odds. This girl yells terrible things to her and makes fun of her in front of others. One of the girls has pushed and shoved my daughter several times. But when my daughter and the girl were alone she would act like a friend. My daughter did not want to get in trouble at school and would not respond or defend herself. Never in a million years would I think that she thought that way...so I am glad that I really listened to her. She also informed me that she looked forward to being friends with this girl again (I did not like this!!!) She would come home feeling defeated. This time I told her about friendship as I did with my oldest, same lecture. (She does not want to be this girl's friend anymore...yeah...this is a tough but necessary lesson) But, this did not resolve the pushing problem. So I decided to tell her it is time she defended herself. I showed her some defensive moves and told her to do back to the girls what they did or said to her. I also told her the boundaries at which she could use this...if she used them as self defense ONLY then I would support her 100% at school. Otherwise, she would face punishment. This was all she needed. Her confidence in herself grew and she refused to be bullied. She told the girls "no".
One daughter "survived" this with a mental lesson only, the other with the mental lesson but more importantly the support she needed from her parents that it is OK to defend herself and how to defend herself.
I hope my stories help. Good luck.