Are you *certain* that he's accessing YOUR 'it works' page?
Have you tried a php that just does contains this code:
Code:
<?php
echo "Yes you have reached the right page";
?>
With nothing fancy going on, it might just work.
If it doesn't, something really strange must be going on. Short of using his computer yourself, I imagine it would be very difficult to figure out what exactly. It could be a firewall thing where he's accessing your site from some over-protective or badly maintained network. It could be a mime thing where somehow his machine thinks that a PHP file is something else and cannot render it properly. It could be a cache problem where his browser refuses to reload the page and keeps referencing the cache (which has an empty file).
If you can use his machine, you could install firefox and firebug and learn a lot more about what is happening. Firebug would let you see request headers and redirects and that sort of thing.
Have you asked him to try a different browser?