He will need to edit class templates, htaccess and stuff.Get RevCMS. Then open the config file and change it from False to True.
<?php
$siteinfo = mysql_query( "SELECT * FROM `site_info`" );
$sinfo = mysql_fetch_assoc( $siteinfo );
if ($sinfo['maint'] == "1"){ '// 1 = Offline, 0 = Online
header ("offline.php");
}
?>