2 things to note:
1) How badly torn was the $2 note? - If its only a small chip, then what's the big deal? The cashier, being a PRC FT probably didn't know how to judge or most probably would not give a damn (as with all PRC FT) and just follow her boss instructions not to accept torn notes.
If the note has a large corner torn out of it, the cashier has the right to reject the note but then again.........
2) How did the cashier (in what tone) tell the customer that they cannot accept the note? I can safely say that the cashier was rude since she is a PRC. Somemore she does not understand dialects so she probably could not communicate with the customer. If it was a local cashier, I'm sure she would have handle it/communicate it better. And I can also safely say that if it was a local cashier, the customer would not have slapped her.
I have seen people arguing over torn notes but I have never seen anyone slapping the cashier before. I can again safely say that the customer is probably thinking: "Who the fark are you to tell me what kind of notes can and cannot be used in my own country?"
It's like going to a hawker stall to order "Lor Mee" and the PRC behind the stall ask you back: "Shen me shi Lor Mee? Ni ke yi jiang zhong wen ma?"
At that point you would probably replied: "Wo yao mai yi wan Lu Mian" - and then pick up the whole tank of hot gravy and pour it all over the PRC.......followed by asking: "Ni xian zai zhi dao wo yao mai shen me le ma?"
Muah hahahahaha.............