okay two things
first. he still has nails....they're just black, which even if they had done something to his nails, it'd take a couple days for the nail to become that severely bruised enough to look black
second, yes, your hair going prematurely Gray due to Trauma IS a thing that can happen...but the hair doesn't turn gray instantly in a matter of minuets, contrary to works of fiction, it only effects new growth, not the hair you already have