They will throw out the employee to avoid being sued. It is just a cold business calculation. They have a “no gun” policy for employees and that exists for no other reason than to protect their financial interests.
I would not be surprised to start seeing metal detectors for the employees to walk through and nothing for the customers.
So the employee should sue them for having a policy that, had they not broken it, would have prevented them from defending themselves against death or great bodily harm.
Two can play this game.
So, what has to happen is to make firing the employee more financially painful than not firing him.