To put it simply, I haven't come across an easy way to just "change" the shorname of a user. It usually causes more harm than benefit.
If you we're intent on using his user data, just backup the data/files and copy the, over to the new user account that you can create from scratch/new @ system preferences>accounts(in SL/10.6)>then add account thirst there.
Using that method and an external drive is the easiest way IMO.
Also, I'd recommend running a " reset home folders/ACLs" after finishing the file transfer, to make sure you have correct permissions levels with your user files you just copied over.
To reset the home folder permissions in 10.6, you'd boot to install DVD, run the utilities > reset password utility, and @ the bottom left of it should be a button that says reset home folder permissions and ACLs.
Again, only run that AFTER manually copying the files over from the other user account/external HDD
But, I do believe that the terminal command for you to change that short username should be like this
sudo mv /Users/FRIENDSHORTNAME /Users/YOURSHORTNAME
and there is a space between the end of friend's short name and /
So, for example if I wanted to move my current account "lamer" to the new shot name "coolguy" then you'd use this terminal command
sudo mv /Users/lamer /Users/coolguy
Please don't be logged into the account while doing this. You can either enable root, or create just a test admin acct for the short time.
**USE TERMINAL JUDO/commands at your own risk I take no responsibility for any issues or problems from using my terminal commands, use at your own risk
Silly me, I found this @ work the day after posting your response, it was late last night, I didn't even think to search Apple's "kbase" (knowledge base)
Will give you step by step detailed instructions on how to do it, and how to do it properly.