The only person lacking derech eretz is the employee. I would have left no tip and talked to management.
I have no idea if she had had whatever discussion with some jewish-looking customer (who could have been anyone asking for kosher food, actually) and if so, whether the customer was right (I notice there was some mistake in your order, so she doesn’t appear to be the epitome of professionality), or whether the waitress was. Let us assume, for the sake of discussion, that some customer was rude to her. This is no reason to call an ethnical group “pretty disgusting to me”. The lady would never dare making such a comment about African-Americans, would she. If she did, she would be made to regret it sorely — and she would fully deserve whatever unpretty consequences would be coming to her.