If that's the standard on set, the standard is wrong. It is the most basic rule of firearm safety to treat a gun as though it is loaded with live ammunition until you have verified otherwise. That verification is not accomplished by simply trusting the guy who handed it to you--I don't care if the Command Sergeant Major of the Army hands it to me and swears on a stack of bibles, his pension, and his children's eyes that it is unloaded, I'm checking the chamber before I point it at someone and pull the trigger.
Sure, the prop guy has a lot of responsibility, but so should the guy who pulled the trigger. If the actor thinks he should just assume the gun isn't loaded because a professional handed it to him, then that's on the people in charge. In this case, that would include the producer, Alec Baldwin.
As a legal matter, while I doubt Baldwin will be prosecuted based on what we know so far, I will be shocked if Alec Baldwin or his insurer won't be writing a pretty big check to that poor lady's family.
Also, this sort of thing is why I think it should be mandatory that firearm safety be taught in school. It shouldn't only be gun owners who know how to handle a gun safely.
I should modify that: If it was something other than live ammunition, then perhaps there is scenario where Baldwin is purely innocent. But I am hard pressed to imagine what other than live ammo could kill one person and put a second in the hospital.