You will need to mod his xbox at least temporaily. The new hdd you put into his xbox needs to be locked to his motherboard.
Easy way to do this if you have tools is:
Backup your complete C: and E: to your PC.
Put the new hdd into your xbox, format it using X3 Config Live, and FTP the C: and E: onto the new drive. Leave it UNLOCKED.
Now get yourself a xapter. Screw it onto the motherboard of your mates xbox. Plug in D0, and the eject/power adapter (you can use your own you don't need to buy a new one, and then plug your own X3 onto the xapter. That should all take about 5 minutes.
Boot into backup, this should bring up flashbios on your mates Xbox. select Lock Hdd and then turn it off. Remove the X3, xapter, and eject/power and put them back into your xbox.
Turn on your mates Xbox.........fixed!
Oh yeah, there is also a way to do this via a softmod I think, but i don't know the steps, because softmods are for pussies! LOL J/K