Save off an IPCONFIG /ALL from when the computer is working, and then, when he has an outage, do another IPCONFIG /ALL during the outage and then see if there's a difference.

I had a problem recently where it was the WNCE2001 (suggested to me by this BBS thread) occasionally usurping the DNS on my home network. The DNS addresses, instead of pointing to the ISP, were pointing to the address of the WNCE2001 adapter.

After setting the WNCE2001 to Fixed mode instead of Auto, I haven't had a recurrence.