Edit /webim/js/source/chat.js, line 162
  onThreadClosed: function(_response) {
   var xmlRoot = Ajax.getXml(_response);
    if( xmlRoot && xmlRoot.tagName == 'closed' ) {
     setTimeout('window.close()', 2000);
   } else {
     this.handleError(_response, xmlRoot, 'cannot close');
   }
  },
Replace setTimeout('window.close()', 2000); with your code, for example:
window.location.href = "http://...../done.php"
PS: Read 
http://mibew.org/forums/index.php?topic=329.0 to compile your javascript.