Can he ever use both computers at the same time on the
internet? If it is just a hub and it is connected to a modem
then the problem may be due to an IP address conflict.
Some modems only provide one IP address so whichever
computer gets it can access the internet. Turning the modem
off resets it. A router can use NAT to create more internal IP
addresses but a hub won't.
Next time it happens do an IPCONFIG /ALL on both
machines. If they have the same IP address, that's the
problem. He'll need to buy a router.