I hate to say it but I think this may be one of those scenario's where your daughter needs to figure it out with the other girl without parental interjection. If you do say something to the other girl's parents, and then get together with them again, your daughter may be in for a lot worse treatment. Around that age, I was being teased mercelessly by a few peers, or "friends" as I called them at the time, and after my parents got in the middle, it got a lot worse for me. I think I cried every night for about six months after, and the teasing got worse and worse and went on till high school. I would try getting them together again and see what happens.
This other girl sounds confused and hurt about your daughter no longer being at school. Also, this is girls and how they play the game of friendship particularly tweens. BFF's one day and enemies the next. Parents don't always hear about it though, and it's wonderful that your daughter trusts to go to you when things like this happen. You may break that trust if you "tell" however, becuase of the reprocussions for your daughter. Even though No one blames you for wanting to help.
Explain the possible reasons this girl may be lashing out to your daughter, and discuss with her different scenerios and how to handle them.
If you do try to get together with them again, check in more often, and provide your duaghter with a secret code, where she can secretly let you know it's time for parental intercession. Then you bring your friend, the other mother, with you to check on the girls and so she can see first hand what is happening. Your daughter will feel safer, and more comfortable to know that you have her back if she needs it. She'll also gain quite a lot of confidence if she is able to handle it on her own, which is a lesson which will help her for the rest of her life with regards to female relationships. Lets face it, we're not that easy to get along with, and it's not at all like boys or men who duke it out or yell for five minutes and then go back to the way things were. I hope this helps and good luck. This will make your daughter tougher for later on in life, eventhough it is so hard to deal with as a tween. I remember what it was like and I hope the best for her.