That's how I measure my cuts too... I hold up the entire 8-foot piece of plywood horizontally in mid air... eye ball it just right, and then just skip cutting it and nail it overlapping. When you've been building as long as I have, you learn this skill.
Actually, and not to be a dick, but to actually do this guy a favor... someone really needs to call the code inspectors on him. Someone will get killed in that thing when it collapses on them, or at least gash the shit out of their hand and possibly disable themselves when they touch one of those pounded-down 8-inch nails sticking through his walls.