I would help her out, despite whether I thought that she actually deserved it or not. I think that it's important to set a good example for people like that who are often rude to others, especially because they usually have a lot of problems in their life. If she doesn't appreciate it then that can't be helped, but at least I'd have some peace of mind knowing that I did the right thing.
And besides, anyone who would be mean right back at her would only be sinking to her level, so they shouldn't really have a right to be complaining about how mean she was to them in the first place... They're not acting any better themselves. XD Ahem!