It's sad but often true that kids can be pretty heartless toward other kids - sounds like your friend is getting more than her share at the moment.
You're already doing the most important thing by being ready to listen and doing your best to be supportive, even when no one else seems to be doing so. You said you don't get to see her much, so these ideas may not be possible, but:
Could you include her in a group project for a class or, possibly even better, for an after-school activity that would be enjoyable and would give others a chance to get to know her and recognize her positive qualities?
Could the two of you organize a group to take part in a community event of some sort? Get a team together for a benefit race, find some friends to volunteer at the local animal shelter, whatever would give her a chance to mix with and make new friends!