I realize I'm 9 pages late into this thread, but it is very possible that a spotty wifi connection can cause speedhack-like responses server-side. If you're dropping a lot of packets, you'll show up on the server as moving very erratically. If your connection is bad enough, you'll literally jet across the map once the server catches up with your slow connection.
Seriously, this is Wireless Networking 101 stuff here, guys. Anybody who's played FPS games for any length of time knows this. Source Engine is notorious for being particularly vulnerable to lag-related issues like this. Bott mentioned cl_predict 1, however this doesn't always work as it's intended to. It breaks just after nearly every other engine build update, along with a dozen other server-side commands.
Just saying, it's perfectly reasonable for somebody to suggest that a spotty wifi connection can cause movement that would appear to look like a speedhack.
However, this just goes to show the importance of recording a demo. A demo will show us his movements, which you can make reasonable assumptions on his packet loss (if any) on.