myself I'd be telling myself 'run Forest run' but a couple ideas.
if the cabinet is literally falling apart & dropping parts I wouldn't even feel safe with a 'banner sigb cover' .. Just me, I'd rather not be losing sleep because of worry that a piece might fall out the bottom or whatever.
does this business owner own the building? Or rent/lease? If he is a tenant ask him if the property owner is aware of the serious risk & liability and maybe there is an opportunity to replace the entire sign structure.
if the business owner is the property owner I would encourage him to allow you to properly disconnect,decommission & remove this accident waiting to happen.
if he is unwilling or unable to replace the sign financially then offer him some less expensive options.
if he is hell bent on using an unsafe structure I wouldn't touch it for any amount of $,but you may want to offer him a cover or replacement panel for his own installation (this would not be my 1st choice) but if he is going to disregard responsible advice & proceed anyways,,,write up a stern disclaimer stating upon your professional examination & recommendations the sigb should be decommissioned & removed & that cover or replacement panel are provided for customer installation against your professional recommendation.
just something to clearly state you have informed him that the sogn is a danger & that he is disregarding that advice & proceeding on his own. You can't protect some people from themselves.
unfortunately after years of servicing electrical signs I will put it this way. I will not park under one unless it is one I made many yeats ago...& I have no problem making a wide circle in my walking path to avoid walking under one whenever possible. I've seen HUGE pole signs (& every other type of sign) suspended 60 to 90 feet in the air, perched upon top of a pole, held there with TWO loosely tightened bolts lol. I've seen lexan panels duct taped from the back to hold them in. I've even seen a 60 ft pole sign 100 plus feet from the building STUFFED with garbage??? I have always scratched my head over that one but it was immediately obvious why it was not illuminated evenly.