Something like this happened to me when I was in 7th grade. We moved from Virginia to California before 6th grade, and apparently I "stole" this one girl's best friend. We were friends through 6th grade, and during the summer, but once school started for 7th grade, the two of them wrote me these horribly cruel letters and put them in my locker. It was weird, because if the other girl wasn't around, the friend in question treated me fine. It really hurt. Good news was that in 7th grade, there was a new school, so I was able to make new friends. But I came home crying many days. They even forged another girl's signature!
I don't know what to tell you to do. I wouldn't let my mom intervene, but with 6 year olds, it might be a good idea. The girl I was friends with initially was having problems at home-- her parents were divorcing, and her dad was a scumbag. Doesn't excuse her behavior but it explains it. The other one was just a jealous, vindictive witch. But 6 year olds learn that from somewhere. Maybe talking to her mom would help?
So sorry for what your daughter is going through. Ouch.