Sounds more like hardware, driver, connection manager, browser add-ons or application update setups to me, not a virus or hacked system(unless it's rooted). Doing the cleaning steps won't hurt though.
I'd guess Ethereal wasn't able to capture wireless traffic because he's using a utility like IBM's Thinkvantage connection manager that doesn't play well with Ethereal. You could also use Wireshark - that's the current Ethereal replacement. Packetyzer is also a handy Wireshark enhancement.
You can also use NETSTAT -b -n to see which apps are connecting to where and shut them down one at a time to see if any are culprits.
A once per second browser connection seems unlikely to slow a network significantly. Check his TCP settings and look for duplicate MACs or IP addresses. You could even temporarily swap out the wireless adapter in case it's defective and looping hot bits to the network.
Lastly, he might have installed a bad driver for the card. IBM's thinkVantage update indicated an upgrade from Intel for my wireless card. Microsoft whined about uncertified driver, but I ignored that, resulting in big problems with the wireless slowing and dropping. Thank heavens fore system restore
How does his PC fare on a wired connection?