Was he riding the brakes all the way down the hill? That just builds up heat.

I assume it works like caliper rim brakes: it's better to hit a brake hard, then let it off completely to cool a bit. If the hill is too steep to let off braking for more than a couple of seconds, it's good to be alternating front and back brakes on the straight sections. If it's extremely steep and also quite long, I'd stop part way down to cool things off, too.