If he is unfamiliar with, or not trained properly in the weapon he is using, he should obtain a Sig or a Glock. They are good weapons and easy to use. In any case, as has been mentioned, he should carry a round in the chamber and be thoroughly familiar with the weapon.
I'm confused. After attacking a customer with a shopping cart filled with money and having to be saved by Gecko45 the OP's brother Leeroy Jenkins was then attacked by a real ninja with throwing knives in the parking lot only to again need Gecko45 to come to his rescue quickly pulling out and fumbling with his firearm dropping it, frightening the ninja, who then like the ninja he is vanished?
No, the whole story stinks. I was in grocery stores a lot for 25 years while in the POS business and I never saw a cart full of money. It went from the registers to the office usually one lane at a time and was picked up by the armored car company or stored in the safe.