Some would say it makes no difference....
I'd say that this will mess with the fishes' circadian rythms. They need some time in almost pitch dark (with a little "moonlight") to feel safe. Fish often change colors drasticallty in the dark--neon tetras are perhaps the best example of this. If the light's on in half the tank, the fish probably think they're under a mangrove overhang in one half of the tank, and in full river in the other.
Also, you'll notice if you turn the light off and put on a weak overhead lamp that the fish calm down and swim quite slowly in the dark (nocturnal fish exempted). My tetras and danios almost stop moving. The danios just rest at the water's surface.
So, I'd say that your friend's idea is ingenoius, but not natural.