If it is modded I would try to reinstall the M$ dash. If you are chipmodded you can just run Slayers and do a return to stock. But I'm guessing that it is not modded. There is no password cracker, and if there was, you would have to be running a hacked bios to use it, which would get you banned as soon as it cracked the password.
Shit, nevermind, that won't work. It's saved in the Gamertag, not the settings. You can't move a gamertag even while modded, the only option I can think of is to delete the account and do a recovery, but if you don't have all his info, as far as credit and address, I don't think you can re-activate the account.
A memory card is the only spot that you can move a gamertag to also. So you might be able to try and put in on the Memcard then delete it from the HD, then copy the Memcard back onto the HD, but I really doubt that will work.
That really sucks.