I am sorry Stoke, but I do not think its a DNS problem, as he said he can do a traceroute just fine, which incline to the fact that the dns reported the correct ip from the hostname to him.
However, the problem might still be IS. It is one thing to do a traceroute to a site, and another to visit a website on the same site. It does happen that icmp packets follow one route, and port 80 connections a different route, as most major ISP's use transparent proxies for http/port80 connections.
Now lets say the route the proxy follow is different than that the normal icmp packets would follow, and the connection the proxy would follow to that site has a problem. Now we know why he can traceroute to the site, but not visit it via a browser...
Unfortunatly I dont think you can fix this, except report it to your isp, who must report to IS, who should then follow this up.