I mean, they had to compliment her first. She's a credit to her people, after all. If this story is indeed true.
I'm not surprised by this shit, but then again, as a black dude, I'm very much aware of the racial issues minorities face in this country from first hand experience. Not all racism is displayed with anger and "nigger this," and "nigger that." A lot of it is subtle, "You're probably a good person, but I still don't like your people, nor would I trust you to hold my purse/wallet, or come to my house." Subtle racism is frustrating and hard to fight against. There are so many culturally "accepted" stereotypes out there that most of the offensive things being said about minorities aren't at the extreme end of racial epithets.
They probably thought they were actually paying her a compliment, and that the rationale behind not leaving her a tip was sound and inoffensive. "We normally tip, but not black people. Sorry. Should have been born white. Good luck trying to pay your rent this month, though. All lives matter!"