Wobbly, the slide cannot be removed with the magazine in place, so it was out when he was field stripping it. I've been guilty of putting a loaded magazine back in the frame just to see where the magazine and bullets were relative to the other parts, especially the clearance to the slide release nubbin. ONLY if the chamber was confirmed clear AND the magazine removed before he removed the slide will I cut him some slack. But, you made a good point, and if he is new to guns, a very good point.
I'll add to your advice. I clear a carry gun in one area, then leave the magazines and live rounds in the bag in that area (usually an office), then take the cleared gun to the bench in the garage. I don't have any live rounds near the bench. Range guns are cleared at the range before putting them in the truck for the drive home, then checked again before going in to the safe.
Joe