Sure let's assume in this case they were both high and that's the ONLY reason someone ended up in the hospital (never mind the thought experiment of what would have happened if they were high WITHOUT the gun). 

So what about the cases when people are fully sober and buy a gun around the corner and commit suicide, or murder their spouse, or decide to go rob someone, or decide to go shoot up a school. Is pot to blame in every one of those scenarios?