[kh99]

OK, maybe your plugin code could be this (using hook build_navigation_data - and you can disable the other plugin):

if (is_array($navdata['link_mzg2_354']))
{
   $result = mysql_query("SELECT * FROM ajax_chat_online");    
   $num_chatting = mysql_num_rows($result);    

   $navdata['link_mzg2_354']['text'] = "Chat ($num_chatting)";
}