You could do a roleplay with her
and then practice what her response should be when someone is mean to her .
If she feels most comfortable with ignoring rude comments, rather than responding, then that is what she should do.
She can practice blocking out someone that is being rude,
maybe sing the ABC or a funny song in her head every time this classmate talks
or she could have a rehearsed comeback for the next time this classmate says something to her, so she's not unprepared
it could be something like
"You're being rude, do not talk to me anymore" and then turn your back on her and ignore her for the rest of the class
Every time she tries to say something, just turn and walk away
tell her that its ok to be rude to someone that is rude to you first, so she knows that she wont get into trouble.
We teach kids to be polite and kind but they should know that not everyone deserves kindness, and being kind is not the same as being a doormat.
Does she have any siblings?
Having a brother and lots of cousins definitely toughened me up against mean comments, by the time I was in school I had already heard all of the worst insults, I knew that I had bushy eyebrows and a big head so I knew exactly how to come back at them 😂