I don't know whether it would work but it would be illegal as the subscription is for one residence only.
It's marketed as something that will allow people to watch their own cable/satellite connection remotely, via the internet, which I imagine would be OK under Sky/Virgin's terms. But it wouldn't be allowed for someone who is permanently living elsewhere, unless he has a subscription of his own.
He'd also need to make sure the internet connection at both ends is fast enough, and the data transfer limits aren't exceeded.
The best option is obviously to get a proper dish installed and subscribe to Sky properly, I would only use the Slingbox as a last resort if nothing else is possible.