Gum would help. Amputation on the other hand would get the job done too.
I suggest he just take a knife and slice off the nails D:
The whole finger is better, otherwise he might try sucking his fingers and that will make him look gay
I asked my mom, and she said (with a straight face) "Put a small amount of poison on the tips"
She explained it too. "If you try biting your nails you will start feeling sick and your body will do what it needs to survive and instinctively stop biting the nails".
She went on but I was laughing too hard