Seems silly to me to deny your ANY percentage of your brakes the chance to stop you in an emergency. I use both brakes often, and have found that good use of the rear has led to being quicker on the twisty roads. By splitting the braking between the front and rear, you minimize chassis de-stabalizing, and the bike is better settled and set up for the turn. Only using the front brakes has my bike feeling squirrelly and not at all confidence inspiring. Also I find I often over brake for a corner when only using the front. Using both it is easier to modulate my corner entry speed.
In an emergency braking I also use both brakes and if the rear locks up, it is still controllable.
I would tell the new rider to practice using both brakes so he or she understands and knows how they react. Telling someone to ignore any of their braking potential seems like a bad idea to me. Just my thoughts.