Coworkers calling SIL “retarded”

Help!! So my sister in law has autism, and she works at a little restaurant doing dishes. She was SO excited because she had never been able to work before. She has been there for almost a year now. Well, one of her friends told her dad that the girls were being extremely rude to her, one even called her “retarded” and told her she needs to “learn how to wash dishes faster because she is doing a shitty job”

So her dad isn’t sure the best way to go about this situation. My SIL never told him people were so awful to her because she loves her job and doesn’t want him to make her quit or anything. But her friend said she caught her in the bathroom crying the other day.

How would you handle this situation? He isn’t sure what to do or how to make this better. The girl who told him said that the manager knew people treated her this way, but never cared or said anything.

Thanks