As a high school teacher this can get VERY messy. Unless you know the parents well, do not contact them. I would not recommend approaching the bully either. Make an appointment with the school and take printouts/screen prints of any facebook/msn activity.(before the bully deletes it) Also save all texts. Make sure your daughter does not respond in any way, because in these cases the bully often says she is a victim too (and sometimes this is true) you also need to make sure your your daughters friends are not "retaliating" on her behalf. I would recommend your daughter finds other friends, this is a pattern for many teen girls and I often see the same girls/ group to deal with this bullying over and over. They are enemies, they make up and then it starts again.

As an aside, has she "unfriended" this person and blocked them? I am always amazed how many teens have ppl they are fighting with allowed access to their social networking!