I would suggest that your daughter needs to handle this. She can do so with your help and support and direction, but she needs to handle it.
My first suggestion would be that she may want to scale back or severely limit her social media time. Set everything as private as she can. Or delete it altogether. Certainly block this "friend".
If she is old enough to have a FB page, then she needs to learn how to manage it. Otherwise, delete her account. This won't be the last time someone hurts her feelings via social media. Kids can be very cruel.
Teach her to be wise about how she posts, what she views, and how she chooses friends.
My daughter doesn't even HAVE a FB. She is 12, 7th grade. Son is 15, and does, and has since it was "legal" for him (age 13). He was told from day one, that if we find anything inappropriate by him OR his friends the account will be gone. To NEVER post anything unkind about anyone and not to get down in the dirt when others do. If they post like that on his stuff, to delete it immediately and possibly removed them from his list of friends. He knows (and can tell them) that if inappropriate things are posted, they will be removed, per his parents. And I do check his account. I have the password info.
She is just starting out with it (I assume) so maybe she may want to rethink it. She may not be ready for it if she is already wrapped up in drama. My son is pretty non-drama oriented. Daughter is the same. I don't expect a lot of stuff from them, but what their friends' post could end up being another matter.
If your daughter is really hurt, talk with her. Let her talk to you and you LISTEN. Offer thought provoking comments to help her figure out what her options are and how she wants to handle it. But let her do it.
If you KNOW the parents, and by that I mean you have their phone number and can call them up and speak with them on a first name basis... then perhaps just giving them an FYI that they might want to check in on their daughter's FB account, because she might be posting things they wouldn't approve of... that MIGHT be ok. But I certainly wouldn't call them up and tattle and say "Your Janie called my Betsy a ____ on FB!" And if I didn't already KNOW them and have their number already in my contacts list, I wouldn't call them at all.