Although, if he shield bashed an NPC to calm them down/cause them to stop flailing/creating noise, etc. As a GM, I would have ruled he was attempting a nonlethal strike, and applied the -2 to the hit and had the NPC go unconscious, not dead.
I asked specifically if hw wanted to go non lethal and he said "Nah" really wanted him to be quiet. If this was a 1 off I would have but it is his third time doing this type of thing. Making him redeem 6 straight creatures without violence might be the answer though.