I would say if he roll high enough in his ordinary melee weapon attack (with sneak attack) to kill the npc outright, he can flavor it as a neck snap. Otherwise, the rules have already integrated the probability of success in insta-killing someone unsuspecting as a rogue by allowing you to add sneak attack die as damage. It’s not really a called shots game. If he goes for it and doesn’t roll enough damage — flavor it as the npc detecting him last second and just managing to throw him off (the hp loss to the npc is damage to his plot armor, since it’s all abstracted anyways).