People are confusing cars and coaches/trucks here, on a heavy vehicle if you tried to use only the brakes you would soon come unstuck (as this driver did). Depending on the type of vehicle there is usually an engine brake ,which works better if the engine revs are higher (unless its a transmission type retarder) so you have to use the gears if the terrain dictates this (hilly roads).Even with no engine brake if its hilly you should use the gears to help keep the vehicle under control using the engine braking.On a modern car you would probably be o.k just using the brakes (don't agree myself though) most of the time.
Also , was the vehicle maintained properly ?
And I think Scunner touched on, you can instruct someone till you are blue in the face, but when the driver is on his own you can't make him do anything.