Hi there, Leon.
I used to run into this kind of thing all the time when I worked for the government: truth is, nobody wants somebody else's name on *their machine, even if it is a buddy, right?
You have a couple of tasks ahead to make this happen, ok? I suggest creating a parallel new account, just for you, and archiving his entire user (temporarily.) Do you remember the original name of his account as well as that password? You'll need it, so jot it down...What I would do, Leon, is set your current account back to his old name and password first. You'll see why...
Here's what to do: Apple |System Preferences |Accounts. Now click the + sign to create your new account with your desired username and give yourself Administrative rights (the checkbox.)
When you are finished and save it, click on Login Options and make sure there is no check in 'Automatically log in as..." Also, select List of users under 'Display login window as'. Now, close System Preferences and Logout.
When the Login window appears, log in as the new user you just created, go back to System Preferences |Accounts and highlight the old user account and click on the minus sign. When asked, (since this is your first time) select Archive or "Not to Delete Immediately" when the OS asks you how you want to delete his old account....An archive will build and save all his docs, settings, etc. and place it in a Deleted Users folder (which we'll clean last...)
Now, open a browser and search for a freeware download called changeshortname. Download that to your desktop (for ease of use) and gather your note with his old acct name and old password. You will need his shortname (the computer name with *no spaces at all) and his original password, likely. Read the instructions before you try changeshortname. It is really easy to use, but mighty!
Last off, Leon, just go to Spotlight, search Deleted Users, go to it and trash his user's .dmg , Option+ Empty that Trash, and Restart. It wouldn't hurt to download OnyX and do a little housekeeping next.
And there you are: you have reclaimed disk space, a properly named PowerBook, the account and password of your choice, and have an optimized system that's right as rain!