I so feel for you DGD! And for you and her parents, as well - it must be so hard to have to "watch" this going on.
I think a lot of good advice has been given here. I want to emphasize the idea of her joining extracurriculars - ones that none of her classmates go to, if possible - and making new friends there. They won't be at school with her, of course. But maybe she could get together with some of them on weekends and so forth. It may not make up for the loss of her school friends, but at least, she'll know she has friends/that she is still likable. To me, the worst part about this is the fact that a child can lose self esteem cuz of it, so that's why I'm saying this.
Unfortunately, though schools can stop bullying, they can't force people to be friendly and so forth. If it doesn't blow over by the end of the year, DGD's parents may want to consider changing DGD's school, if possible.
As for the mothers, I agree with you. Sadly, I've known mothers who openly go along with or even encourage this sort of thing. Either they're just glad it's not "their DD" who's the target or they did the same, themselves, when they were girls and think it's ok. Or maybe they don't have much control over their child and so don't want to deal. Some may try and their DDs just aren't listening, but others avoid the issue altogether. All the more reason, I think, for changing schools and getting away from these people.
My heart is with your DGD and I wish her all the best!