I don't think you need to pierce anything to fuck someone up with a rock. People get concussions while wearing helmets, this is just more extreme.

Medieval fighters used maces against armour. I'm not sure how well a combat helmet works as a hard hat against thrown debris (since they're designed for bullets/shrapnel rather than blunt force trauma), but I think getting concussed enough to die is possible. Or maybe his neck got broken, idk.