Which components affects downhill speed?
It's really simple, gravity/weight moves you forward, and drag/friction keeps you from going fast.

He can't do much about the weight difference, short of drinking more beer or getting a heavier bike.

But drag and friction is a different story. There are many things slowing you and your buddy down, the largest factor being wind resistance. Wind resistance is related to the square of your speed, so twice as fast means 4 times the resistance. Secondary factors would probably be your tires (knobby vs smooth, soft vs hard), and even your hubs. I once had an old hub serviced and started keeping up with my downhill-coasting buddies.