What happens when your friend turns his canister filter off is a die off of the bacteria that are the reason for the filter. I'm guessing he doesn't understand the process in the filter. Yes, it polishes water, but its main function is to support a bacterial colony that feeds on the toxic wastes produced by the tank. Turn it off, and it doesn't work.
It costs pennies to run it 24/7, but it's the price you pay for having a fishtank. Depending on what fish he has, there could be cheaper alternatives to a fluval canister, but they are very energy efficient.