Why would'nt this work?

This doesn't account for blinking.

If your friend blinks, they won't see the light, and thus would be unable to verify whether the method works or not.

But how does he know when to open his eyes? He can't keep them open forever. Say you flash the light once, and that's his signal to keep his eyes open. Okay, but how long do you wait before starting the experiment? If you do it immediately, he may not have enough time to react. If you wait too long, his eyes will dry out and he'll blink.

This is just not going to work. There are too many dependent variables.