Realistically, that likely won't work simply because the holes in the 2x4s are likely less than 1/2" and DVI, nor even HDMI will fit through them.
Your buddy has to open the walls - simple as that - if you want to hide wires. Then you should run ALL the wires necessary. Component video, HDMI, VGA, composite video... maybe even a second HDMI run. Cable should be run to the (duh) cable box, not the plasma.
Audio feeds for later use make a lot of sense, just in case.
But, if the home is new, then now is the time to do it before custom painting is done to the room.
It is unfortunate that so many new home builders are 100% clueless about proper wiring that is necessary to run within an A/V installation. Your buddy is SOL on the deal and even worse - he PAID for it! Proper wiring is impossible from almost all builders unless the buyer knows everything that is exactly needed.