Forced to pay 10% of R1300 and something tip

Hi I just want to find out how can you allow such day light ******* to force people to pay 10% of their bill as a tip. My Aunt went to savanna mall in polokoane with her family, her bill was R1300 and something, she offered to pay R40 tip and the waitress refused claiming it is compulsory to pay 10% of the bill. She ended up pay R1500 and something. Worse part the waitress left with the slip the only proof of payment she has is transaction from the bank account. This one I won't let it go until the money is paid back to her account. If it is compulsory please make a note on the menu or put a board by the entrance. I always go out for lunch at spur festival mall and mall of Africa, I haven't came across such nonsensical issue. Hope this will be sorted asap.