I hate to see kids hurt like that!
PO is right, though...it's like the new "moron" or "idiot." Now the ante has been upped--it's the "b" word, slut, whore, gay, etc.
When my children get their feelings hurt, I remind them that only stupid people say things like that, and they shouldn't care what stupid people think. (of course, they do, but it makes them feel better to hear me call those kids "stupid"
) Then I ask them who their "real" friends are. When they answer, these are the kids I invite over to spend the night, etc.
Also, responses only tend to egg on the brats, so the best thing your daughter can do (if possible) is to act like she isn't even affected, to walk off, and at the risk of sounding heartless--to NEVER let them see her cry about it.
If it keeps up for any length of time, then I would visit with her teachers and the principal.