Are you saying he has lost his administrator status for his account? How did these accounts get there without one admin account?
Restore Admin Status to User Account - Snow Leopard
Boot from the Snow Leopard DVD to enable the root user:
After the installer loads select your language then click on the Continue button. When the menubar appears choose Reset Password from the Utilities menu. Select the account to reset password, choose System Administrator (root). Enter a password and verify it. Save the password, then exit the installer and restart.
At the login window, or if you have it set to auto-login then log out to return to the login window, choose Other... and enter root (all lowercase) as the name, then the password that you set. As root you will have full access to the system including granting admin rights to the accounts that are standard at the moment. You also will see some additional folders on the hard drive you couldn't see before. Do not touch these in any way or you could hurt the OS. Access System Preferences->Accounts, select one of the non-admin accounts and check the box next to "Allow user to administer this computer," then log out and into the account to which you just gave admin status.
To disable root go into Accounts preferences from the admin account, click Login Options at the bottom of the list of accounts. Click the Join button next to the words Network Account Server..., click Open Directory Utility, click on the lock icon and authenticate to make changes. Under the Edit menu, choose Disable Root user.
Consult this Apple support article if you get lost.
I did not write the above as it was provided by another user.
How do you login to Root and change a user password? The user has only one account, his, and that is the only admin account.
If you have the 10.6 install disk hold c or option/alt key and boot off the 10.6 disk and use the Password Reset under the Utilities menu, you will need a wired or built in keyboard.
If you dont' have the 10.6 install disk, then hold command s and boot the machine into Root.
type and press return after each bold command
mount -uw /
(find the name of the account)
Enter the new password twice followed by returns then
Then reset the keychain in either method you use.