Good question, I am not a lawyer but ill give you my two cents.
First thing is does he own the store. does the business belong to him?
Is it a private business or run in conjunction with state or federal intervention?
If he owns the store he can have a gun wherever he may choose in the store. its considered private property and can be treated like a home.
If he doesnt own the store but knows the Owner
of the property and building and has explicit permission from the owner to have a fire arm in the store then i would say that is also perfectly legal.
Not sure if i helped at all, again im no lawyer so take it with a grain of salt.
Look up : State of Maine Castle Doctrine (Stand your Ground Law)
It may be covered in there