If you find out his browser you might be able to get them to look at their settings to see if it is enabled in his browser, but I'm not familiar with where it necessarily is in all browsers.
You could ask them about firewall settings... or probably the most definitive test would be to create a test page for them to go to. The page should have a submit button and a textarea with the value:
<?php echo((isset($_SERVER['HTTP_REFERRER']))?$_SERVER['HTTP_REFERRER']:""); ?>
If you go to the page and submit the form, it should show the current page location.
Ask him to do the same and if he does not see the current page name, that is the issue.