I don't see an unauthorized purchase in this case?
There's no way he's getting his money back, regardless of the type of card he used. He authorized the purchase, and there is nothing suspicious about the bill as far as the amount goes. His story would basically be: I was so intoxicated at the time, that I didn't review the bill before signing it and did not obtain a copy. Yet I was sober enough to recall the exact number of drinks I ordered, there prices, so I know they overcharged me a 100 dollars. No card is going to get his money back.
I only install the stuff so the full functions of these POS terminals I am not familiar with. but I do know that depending on the system transactions can be changed/altered or the clerk can simply overcharge the customer and pocket cash from the register.