Given the sound the bar made, it obviously wasn't very heavy, so the beating probably looks much worse than it was, but that was still overkill.
Honestly though, those women had no right to get aggressive at all. If the authenticity of a bill is called into question, that doesn't make it your fault it's counterfeit, it just makes it counterfeit, and jumping over the counter to assault the man saying it's counterfeit calls all kinds of questions up. For example: Why did you react so violently if you knew the bill was authentic?
It would have been much easier to ask for the manager, but instead they jump the counter and assault the cashier? One of two things happened here.
A) It really was counterfeit, they knew it, and wanted to get it back so as not to be charged with counterfeiting/knowingly possessing counterfeit money.
B) They, too, have serious mental problems and their first instinct after being told their money was no good, as it was fake, was to try and beat the cashier into believing otherwise.
Either way, they got what they deserved.