Might be a bit silly but if he is already logged in he can do it through CMD...
Go to Run then type CMD.
Command prompt will come up.
Then type net user. It should come up with all the account names on that computer.
Then type net user (username) then type * (Shift F8)
This should give him complete control over the password for a certain user. This WILL work with other accounts on the computer.... This is how easy it is with XP to " borrow " someones account! Don't forget you do have to be logged on to the computer. You might be able to change the root password if he has an original Windows disc (I know you can do this with OSX)
You can use password recovery software which will try and guess it. If that doesn't work did he create a password reset disc? If all fails then he'll have to perform a complete format. If he has to do a complete format there are ways of getting his data off the Hard drive. So when he formats he can just put everything back on his computer.