Pat his computer down for viruses while you're at it too. Could be infected with something that is serving content from his computer.
One thing I've seen before on lower end linksys and dlink routers is that when someone is downloading multiple torrents at a time, or a single torrent/file share with a lot of connections to it - even though your cable providers bandwidth is not being saturated the router is bogged down from the multiple connections. Solution - change file sharing settings to limit peers/connections per file to less than they are now.
Otherwise, again it may not be your downstream that's saturated - most ISP's these days give you 5-10 times as much down speed as they give you upload speed. When sharing files upload speed gets saturated fast (should set speed limits on any file sharing programs). You come along while someone is sharing/uploading a torrent at max speed and your browser barely gets it's share of the communication pipeline enough to tell the website what page it's trying to look at, what packets it's received of that page, etc.. your connection seems slow but it's often your outgoing rather than incoming connection that's clogged.
But between individual file sharing settings and router settings you should be able to figure things out. Otherwise you might want to tell this guy to get his own internet connection.
