N., this reminds me of the awful thing that happened some years back when a mother and her daughter pretended to be a boy interested in a girl. They got her hooked on this "boy" and then proceeded to trash her. The girl committed suicide. The mom was taken to court over it and got off since there were not enough cyber-bullying laws on the books (still aren't, if you ask me, considering the college student who just got a 30 day sentence for filming his roommate having sex with another boy, driving the roommate to suicide). However, everyone knew what she and her daughter had done and she lost all her friends over it. I don't know much more than that - perhaps she ended up moving and losing her job too, I don't know.
I would actually google stories like that and copy them out to give to all the adult parties concerned, including the mothers and the police officer. Tell them that stuff like THIS is the most worrisome part about it. The girls need to take responsibility for their actions and their threatening messages (like "people die from rumors".) That especially needs to be talked about and the girl called on the carpet for it.
There is a cyber case now regarding a man who has tried to ruin people he has gotten on the wrong side of, including former bosses, former business associates, even people he hired to sell merchandise on Ebay. People like him will end up getting more laws on the books, and those who seek to hurt others on the internet better pay attention. Meanwhile, for these girls, their parents need to do the right thing in chastising them and making them walk a straight line. I hope that they will.
Dawn