They do not need to pay for a second Sky subscription.
They can get a second Sky box provided with a “Freesat” card, their receptor in their Sky dish adapted to serve up to four boxes, all installed and working for a one-off payment of (I think) about £85. (You’d need to check the price, that was how much it was when I last looked).
This is probably what a new aerial would cost you, and you get a far greater variety of channels than with Freeview.
http://www.sky.com/he...t-from-sky/index.html
Many local TV aerial installers would also provide this service.