Without knowing what browser he is using, it could be difficult.
First, whatever browser he is using he should clear the cache and check to see if 3rd party cookies is turned on. If he is using an older version of a browser, upgrade it.
If he is using Firefox, I would suggest checking to see if he has the silverlight plugin installed in his browser and that it is activated.
If it is, I would then see if he has an ad blocker or noscript installed and see if you can set it to allow the netflix site in the exceptions list.
Finally, I'd check the firewall if he is using something other than or in addition to the Windows firewall. Some of them block all kinds of stuff. What initially made me switch to Firefox was when Zone Alarm's script blocking locked me out of GMail and I had no idea that was the problem. (this was back when gmail was still in beta and you needed an invite to have an account)