Also, what is the actual error that he is seeing? Does the item just not appear in the cart? Or from looking at your site, I see you popup an "alert()" window once an item is added...Does he always see the alert() window after pressing the "buy now" button?
If the item occasionally does not appear in the cart, then that could be related to your codebase setting. I see your uShopJSI codebase is currently set to the full URL of your classes (http://www.flagline.com/u3/classes/) while your uShopCart and uShopInitialize applets have a relative codebase setting (/u3/classes/). Although the codebase paths resolve to the same directory on your server, some browsers may not interpret them to be the same... so the recommended thing is to use the EXACT same codebase setting for all of your applets... And since you are dynamically generating the page from PHP, it would be safest to just use the full URL as the codebase of all of your applets ... including your "display_cart.html" page. (See http://www.uburst.com/uShop/codebase.html for more information on that.) Note: If the codebase was the problem, however, the problem would be consistent with the browser type being used. So if the problem is randomly occurring on the same OS/browser... then you're codebase is probably not the problem.
Anyway, check the above two items out... and if you don't notice anything, see if you can provide more information about:
1) What is actually the problem being seen (item not adding to cart, no action at all after pressing the "buy now" button, etc.)?
2) What browser(s) are being used when the problem is being seen?