it's way beyond willpower, it's a bad habit and it's very hard to break. don't totally fault him for it; i am sure he'd rather not bite his nails.
reminders and associations:
he can try a rubber band around his wrist and snap it when he catches himself biting his nails. *ouch* it becomes a negative association (nail biting = pain. therefore: don't bite nails) it's simple but it works.
the polish that tastes bad can be a good reminder, that is some awful tasting stuff. that in combination with the rubber band maybe?
idle hands:
he might need something to do with his hands. give him those meditation balls that you roll around in your palm, and or one of those stress dough things. (i have no idea how to explain what those are. like play-dough with a strong rubber covering) perhaps some hard candy too, in case there is an oral fixation thing going on here too.
having too much time to kill will certainly leave more opportunities for him to "eat nail" so that's something to think of as well.
i know all this from experience.
i'm lucky though, i'm a girl and i can wear acrylic nails to aid me in solving the problem. i have such a bad affliction with nail biting however that i have even bitten my acrylics. it is not an experience i recommend. ouch. my acrylics now look good thanks to most of the tips i mentioned above. it's wonderful to have pretty hands, for the first time in my life.
hope that helps your friend.