Extreme Rudeness towards a simple mistake.

A new worker at Spec-Savers. She had made an honest math mistake, I could see that she was not well and the Assistant Manager was overworking her. She looked out of it and had a troubling voice crack. The Assistant Manager said too her, Infront of myself, "Atleast one of us had maths in school" I find this to be extremely rude and needs to be dealt with, Either she needs to set her priorities straight of serving customers and not demeaning employees.