Not possible. HDD are locked to the mobo with a HDDkey stored in the eeprom.
1. Hotswap and do it 100% free. There is risk of frying the XBOX HDD and your PC mobo though. http://forums.xbox-scene.com/index.php?showtopic=496263
2. You could optionally install your chip in his box temporarily (you would need to flash a 1.6 bios to use it on his xbox though) and then install all of the needed files (I don't think there is any guide to which files you need to specifically install, but you may be able to use the Ndure XBOXHDM files). You could also use CASH v1.1 to do this, click the link in my sig and read post #11 for full instructions if you wish to go that route.
3. You could use your modded xbox to put the saves onto a memory card, then copy the save onto his xbox. But you will still need one of the 3 softmod games to launch the save. (chorizo1 listed this method above)
That's about as much as your modded box will help with getting his modded.