I had a similar problem logging on to my wireless network - spent hours tweaking things on the router until I finally discovered it was an AV sender unit (to send my Sky signal to a tv upstairs) that was interfering. Switched the AV sender to a different channel (out of 4 available) and the problem was sorted. I should have found it sooner - the day before I had selected a different channel on the AV sender to try for a better signal and it didn't occur to me that this might be causing the problem!
So has your friend or their neighbours recently added / changed any similar equipment? (baby monitors, AV transmitters etc.)
Vindaloo