The school probably has a policy on bullying and harassment. It might seem odd, but they must. In the US, if they're aware of issues like that, the administration does something about it. Besides possibly caring, if they choose not to handle the issue, they could be looking at bad publicity and/or a lawsuit.
I really believe that she should talk directly to the professor. If that doesn't work, go to the department head. She needs to keep working her way up until this matter gets settled.
As for why people bully? Insecurity a lot of the time. I think people feel godlike by making another person feel like shit. It's a shame that grown people need to do it, but they do. It may be that only one person really has some stupid issue with her, and the rest just follow along. Recently in my office, some girls decided they didn't like one of the male grad students (he did do some assy things, from what I heard), and they decided to be bitchy about everything with him, even dumb stuff like his wife showing up to see his presentation for class.
People are just awful sometimes.