Another thing you might want to find out is what operating system he is using. I've had a similar problem on my Toshiba Tablet PC laptop running Windows XP SP2 with Zone Alarm installed.
Of course, every single one of my other comptuers in my home network work fine - it could be a problem with adware, bad registry values, or a number of other things - as a quick solution, you could have him try the following:
1) Download a program like SpyBot to remove adware/spyware
2) If windows XP, have him goto start and run and then type the following:
"netsh int ip reset reset.txt" which will re-set the vast majority of his network settings on the comptuer (be sure he is connecting via DHCP to his ISP/router which is what the netsh reset command will reset them for).
3) If the above does not work, i'd have to say its something with the software installed on his comptuer...possibly a firewall program that is not functioning properly. (On a side note, I was at a RadioShack today and their demo comptuer hooked up to a cable modem was having the exact same issue which the above steps fixed).
Hope that helps out some...
smartin [at] nocturnalhosting.com