http://www.DOMAIN-NAME-HERE.com:2082/3rdparty/phpMyAdmin/index.php is one way or if the user has SSH access, he/she could connect directly from the command line.
Why don't you setup a phpmyadmin in your client's space...and just password lock the directory. If you want to be correct don't count the space that phpmyadmin takes as space used by your client. It's not much anyway.