Ubuntu :: 3 User Accounts 1 Admin?

I'm installing a new laptop for a friend of mine and he wants 3 user accounts, similair to how he runs his windows setup.

1, an admin account, we have called this account peacemaker.
2. his account
3. an account for his girlfriend.

The problem we have is that if we want to do anything from the terminal that requires elevated priviledges, sudo does not accept his password or that of peacemakers. we have done sudo -i -u peacemaker but it still doesn't accept either password, stating his account is not in the sudoers list.

I'm not a massive expert here, but research brought me to this page:[URL]... But that then just means his account has admin rights, which is what we were trying to avoid. We wanted a setup similair to windows where if you want to run someting with elevated privledges if pops up asking for the admin password. This works in the gui, but not in the terminal.

So in short, my question is, is there anyway of having the terminal accept peacemakers user rights from the his normal user account? If I add the account to the sudoers list like it suggests, does this again just give his account the prilvedges rather than saying supply me with the password for peacemaker.

this is probably not really needed and he can just have his account as the main user, but coming from a windows background, he would prefer the 3 user accounts model (2 normal users, 1 admin)