You don't even need to wait around for dust to settle, I guess.
Stand so that you can only see half the table through the door. If the other half doesn't go, the half you can't see won't fall over.
(And just to avoid arguments that the whole table is in some fashion one whole, coherent object, we can prop a plank of wood that we can see, on a chair we can't see - does the plank fall down when the unseen chair goes ?)
Alternatively, we could explain that the double slit experiment has nothing to do with observation by the human eye - we can arrange it so that light behaves as a wave and then a particle and then a wave again, regardless of whether the human eye observes it.
Isn't the mistake he's making, trying to extrapolate quantum behaviour into the day to day, classical, macro world ?
Could there be any greater conceit, than for someone to believe that the universe has to be simple enough for them to be able to understand it ?
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