That’s odd. Usually I’d advise you (all/both of you) to disable all plugins, and then see if you can regain access to the dashboard. If you CAN, that means some plugin is stinking up the pond. To find out which one, reactivate each plugin, one at a time, until you find the one that causes you to be kicked back off the dashboard. Then you can kill that one. But if you don’t have access to your backend, the only way to do this is to FTP into your plugins directory and rename that directory, fooling WP into thinking there are no plugins. (OR just FTP all the plugins down to your hard drive, delete them from the plugins folder and reupload each one, one at a time, until you find the culprit.
You could also try changing themes (again, FTP into your /themes directory and download all the theme folders, and just keep the /default or /twenty-ten folder, and delete the others. If the site returns when you have only the default theme installed, there’s some issue with the theme.
Failing ALL of that, I’d go ahead and backup my database, and reinstall WordPress.
You just need to delete the /wp-includes and /wp-admin folders, and all the *.php files EXCEPT config.php. Also, don’t delete your .htacess file or your robots.txt file if you have one. Then grab a fresh distro of WP and reupload everything. That should cure it. It is what I’d do if I were paid to troubleshoot this problem. ??