It would be too messy.

Easier way, get a 4 port wireless adsl router from ebuyer, the safecom ones are fine.. he can then connect his pc up to the router via a normal network cable (rj45)... and then his laptop can surf wirelessly.

Or he could get a USB/PCI wireless card for his desktop too and not have any wires trailing about.

EDIT: The wireless adsl router will have a USB socket to plug his modem in to... or he could get a wireless adsl/modem/router and eliminate the need of the USB modem.

EDIT2: Instead of a USB dongle for the laptop, I'd suggest going for a wireless PCMCIA card..as I'm assuming the laptop will either have a type II or type III pcmcia slot in there.