Although my daughter is older than yours, at 9 she had a situation with a girl who had been her best friend since 1st grade. All of a sudden during the last year she would come over for a play date and deliberately break things that belonged to my daughter, she tore the bed sheets off of her bed, she grabbed her journal and scribbled all over the pages with a colored pencil, she would go through all of my daughter's drawers even when my daughter would ask her not to. Finally, she told my daughter that she didn't want to be her friend anymore because she wasn't cool enough and didn't have an ipod, plus other "cool" items. At that point my daughter said that she would like to go downstairs (they were in her bedroom) and discuss what was going on between them with me and the other girl's mother who was having coffee with me. However, the other girl jumped in front of the door and blocked her from leaving saying that her mother would never believe my daughter. She was right. After they left and I saw and heard what had been going on, I called her mother because we were (I thought) good friends and I wanted her to know the situation. I felt this was out of character for the other girl and was concerned for her that maybe this was prompted by something going on at school with her daughter - maybe she is being bullied by the other girls and was taking it out on my daughter. However, she didn't believe my daughter was telling the truth and just kept saying that her daughter is a "little angel." I later spoke with the father who is a psychiatrist and he felt we should all just pretend it didn't happen. Sorry - it did happen and I am not going to put my daughter in that situation again. We have declined any invitations to get together with them and told them why. I don't worry about my daughter sticking up for herself because I have taught her to say no, etc. but at the same time she just has no interest in this "friend" anymore. I think what hurt her the most was that she was told she wasn't cool and that the parents thought she was making it all up. Unfortunately, it cost us a friendship - but then again, what kind of friends were they? I was truly concerned about their daughter and they chose to turn a blind eye - not doing their child any good.
Just as a side note - when my daughter and this girl were in school together, this girl was constantly being bullied and my daughter would intervene on her behalf. This couple had to go to the school on several occassions to speak with the teacher about the bullying issue. But when their daughter starts doing it, they ignore it.
I guess my whole point is that don't be surprised if the parents don't take it seriously. They should because someone has to make sure this type of behavior is stopped. As others have said, you may end up having to distance yourself, like we did.
It is unbelievable how many have responded to you because they have also witnessed similar behaviour. I just don't remember it being this bad when I was growing up - not sure what that says about our society today. Good luck!