He made the right decision in my opinion. If he would have tried to grab the gun things would have escalated. It's not worth being a hero over a little money. On another note that's why you should always carry with a round in the chamber.
I wouldn't do that to protect someone else's money.But it would be tempting to disarm him to take a gun away from a criminal.
What if he got involved in a shooting with the police after he left the store? Assuming he noticed the gun was not chambered properly.