If we had a link to the site, we could at least see what the cookie domain and path were. Or, he can tell us what they are in AdminCP > Settings > Options > Cookies > Cookie Path and Cookie Domain. (Of course, if he can't login to the admincp, then he can't, so a link to the site would allow us to view them.)
Are you logging into the admincp using
www.site.com or just site.com? Try logging in using the other one.