For Electric or glow use a timer.... I keep the time in my head . Tell him he'll have to worry about the fuel running out now. Fuel is fuel whether it's electrons or liquid. I fly glow and can usually get down to a couple mins of fuel left judging by whats left when I empty the tank.

I beleive the lipos have a fairly contstant discharge curve until the end so you won't really know your low until close to the end. The programmable ESC's usually covers those conditions though by overriding throttle and reducing power to the motor giving you a heads up it's time to land