Sky does not come through an aerial but a dish.
I believe sky leave the dish and box with you to do with what you want (hoping I guess that you will rejoin Sky one day).
If he has an aerial in the house then he can continue to use that to watch "normal" TV.
If they do leave Sky they can buy a Freesat card from Sky for about £25 (one off payment, no monthly fee) to let them watch over 200 free channels via their existing Sky box. More here:
http://www.sky.com/shop/freesat/home/
From the FAQ:
Q. I already have Sky digital satellite equipment - what else do I need?
A. All you need to gain access to the free channels available on the digital satellite platform is a Freesat from Sky viewing card, which costs just £25. If you’d like to access the interactive services available, you’ll also need a working phone line. To purchase a viewing card please phone Sky on 08448 244 400.