It is really hard to get people to see the benefits of their privilege. I know it’s something I’ve struggled with myself, as a white cisgender guy. I don’t know what the best way might be to teach people like me.
But I can’t see what the thing with the tip was supposed to accomplish. No one, of any background or setting, could be reasonably expected to respond to that sort of (admittedly minor) aggression with sudden enlightenment, even if they did “deserve” it. Black South Africans have every right to be angry, and a reasonable person should give a victim of oppression some leeway when they act out their justified anger; but that doesn’t automatically make such actions constructive or morally positive. It’s right to be angry, it’s forgivable to lash out in anger, but that doesn’t necessarily mean that you’re striking a blow for the cause.
I haven’t looked up the full story. Did the waitress mistreat Wandile and their companion, like the tannie at the beginning of your story? If Wandile was responding to direct aggression with direct aggression, that would make a lot more sense to me. But you didn’t mention anything like that here.