If the behavior continues, there are a few options.
Your child could walk away and not give the person the satisfaction of seeing her upset. She could then go talk it out with a friend or trusted adult.
Your child could also report the behavior to the adult in charge and let them handle it (hopefully). If your kid feels like she would get more harassment for doing this, see if there is a way the kid making fun of her could be caught doing it. For example, if Johnny always calls Sally fat in the lunch room, see if the lunch monitor could stay in the area and really pay attention to catch him.
I want to stress that your child calling names back or resorting to violence/threats will result in your child receiving consequences. I say let the other kid be the guilty one!