How to change admin user for GUI?

Tue Nov 6 19:43:10 UTC 2012

Hello,

I have installed ubuntu-12.4 for a friend. Since he is not a computer guy and
I will need to do the administration on this computer, the first admin account
I created was an account for me. Then I created a second admin account for
him. We can both use sudo to do the administration on command line.

Since he is not familiar with the command line, he is doing everything with
the GUI. But when he does anything that needs root permissions (e.g. update
the system), the GUI asks for _my_ password.

How can I change this setting so that _his_ PW is asked instead?