The last line of defence/hope is with you
...
Maybe you should qualify suspected know-it-all duffas types by making sure they know the gun to be rented. Simply rack the slide (then locked back) drop the empty mag, then hand him a 9mm snap-cap, and let him load the mag with it, then insert the mag, then chamber it, then decock back into DA mode, then activate the safey ON, then unload the weapon and verify..
Then you would have "verified" what you really knew (rightfully suspected) but, at that point, should have told him "he needs a lesson or 4, before he can rent. Or at least start off with: name the 4 rules of gun safety.." and take it from there, would have at least given you a start point, as too, should I, or shouldn't I, let this guy rent and shoot.
After all, it's your duty to protect the_babe, (the innocents) from any negligent mishaps issuing a weapon, live ammo, and unleashing that combo, loose, without supervision, into the firing range, if he fails the 4 safety rules or demonstration, type gun, test.. NO..?
I mean, he failed every test, from the get-go, and "continued" with rapid firing, and a no-clue, no-care of rules, etc., just this side of total, out of control, un-schooled/trained actions..
OMMV,
Ls