Not casting judgment on anyone by any means since I wasn't there, but I always stress muzzle awareness with whoever I am diving with, just like it is a firearm. I have witnessed a handful of people accidentally shoot themselves or others with firearms by not using simple proper muzzle awareness and handling, and to me the principle is no different with a speargun.
A couple years ago I took a dive buddy out, and he was a little bit below and behind me, as we were both on the way up. I heard a click and saw a spear shoot right between my thighs, fortunately it didn't get me but it was only inches away from a possibly lethal femoral artery hit. He said he didn't know what happened, but if he had only had his muzzle pointed down nothing would have been a problem. I ripped him up one side and down the other, and I am extra cautious of muzzle awareness now and who I am diving with. Sounds kinda like the same thing here, sure hope he pulls through.
Damn, never had anything like that happen. That's wild, I bet your blood was pumping.
Things like that are why on the one hand it's nice to dive with new people and to help newer divers learn the ropes but on the other hand it can be frustrating. Makes me feel lucky there were people out there with more patience than me when I was getting started.