This should be fairly simple to accomplish. Just log into the panel > Users > Manage Users, and create a new user. Next in the Panel > Domains > Manage Domains, edit the domain you've registered for your friend to be 'owned' by the new user you just created.
There, now that user is the only user to have access to that site, and not the rest of your stuff. You can also grant your friend access to the parts of the panel that pertain to her domain. To do this, Panel > Users > Account Privileges, and Grant permissions to a new user. You can go through and check the sections and domains she has access for.
Hope this helps to clear things up for you.
art.googlies.net - personal website