Wireless Signal Strength

I think something in your friend's environment is causing the problem. Does he have a cordless phone? If so, unplug the base unit as a test and see if that makes any difference. I had to replace my cordless phone when I started using wireless. Does your friend share his house with other tenants who might have similar wireless devices?

Is he by any chance using a different display at his house than the one you tested with at your house?

Graphics cards have to run lots of different oscillators (crystals, clocks) at different speeds to accommodate different resolutions and refresh rates. It's not uncommon for these clock rates to hit, say, 2.4GHz, or a harmonic of 2.4GHz. This means that a graphics card that wasn't emitting much 2.4GHz noise when connected to one display, could be destroying the 2.4GHz or 5GHz band when connected to a different display.

If you've been using an internal 802.11 card, switch to a USB dongle at the end of a USB extender cable (or plugged into a hub) that you can position well away from the enclosure and display. Also make sure you've replaced all the PCIe fence slot covers and the case/door and any other pieces of RF shielding on the enclosure.

Someone suggested inSSIDer, but running a software-only tool on an 802.11 card won't show you non-802.11 interference in the band. For that you need real spectrum analysis hardware, such as a Wi-Spy.

Heh, one last late thought: Did you go to his house to look at his problem? Because if not, it would be really funny if it was something stupid like he forgot to reattach the antennas when he brought the box home, and you assumed that was too obvious to ask about...