What kind of POS system is he using? Have you talked to the vendor for such a solution? Most POS systems will have a way to login or validate an employee. So a customer can't just walk up and press a button and grab the cash and run out the door. This would be the "lock", as they have to sign into the system to open or complete a transacation to open the drawer. These systems normally also have an manual lock that disables the register all together, overrides the login or etc.