Must not be on very friendly terms if he won't give you his password. Why the heck would he even leave his user intact if he was giving you the computer? Just ruminating.
While in your account, open a Finder window, go to Users and make a note of the shortname of his user account (the name of his home folder), and then follow these steps:
1. Boot into single user mode (press Command-S at power on)
2. Wait for all the geeky text to settle down and then type:
fsck -fy
3. Press Return
4. Type:
mount -uw /
5. Press Return
6. Type:
launchctl load /System/Library/LaunchDaemons/
com.apple.DirectoryServices.plist
[This is a long command that won't fit on one line of this forum. There should be no space after "LaunchDaemons/"]
7. Press Return
8. Type:
dscl . -passwd /Users/username password
(replacing "username" with his user and "password" with the desired password)
9. Press Return
10. Type
reboot
11. Press Return