Need help with booting a Mac?

my bonehead friend seemingly set 'no access' to his hard drive and restarted the system, and where's the original OS disc? Lost!

anyways, I know Windows based PCs inside out, but have never worked with a Mac. This guy is a graphics designer in a firm and needs the stuff in his Mac very bad,so as not to get fired.

I tried downloading snowlopard v 10.6.3 from somewhere, and wrote in on a double layered DVD, but no Mac, was able to boot from it, not even a desktop. Note that the original .dmg file was 6.3-6.5 gb, but when extracted, it turned out to be less than 1 gig, donno what happened there.

I even tried booting it from a USB drive (since it is an intel powered machine) by pressing the 'option' key, but nothing other than the hard drive showed up.

Please people, what am I doing wrong here? you need a copy of the OS to boot from, so as to undo the 'no access' mess, am I right?