uhmmmm, i hate to say this but he did pay to play already. He already has FCP 6 so i see no reason why he can't share it to a FCP 5 machine? There are plenty of reasons for not upgrading a FCP 5 machine to 6, one little codec shouldn;t stop him.
Anyway, i have no idea if this will work but its worth a try:
See if swapping the HDV QuickTime component from your MacPro with the one on your G5 will bring over that codec your missing. FCP just reads your QuickTime library for these so it should just show up. You'll prob have to restart your maching tho. You can find it here:
/Library/QuickTime/AppleHDVCodec.component