It's not entirely clear what you mean by a 'decoder'.
As Postdog states, you won't get anywhere without a satellite dish. (The signals, which aren't that strong to start with, have to travel 22,000 miles from the satellite so only a properly positioned satellite dish can receive them).
Assuming that your friend has got a dish and cable, then he requires a satellite receiver (which is what I assume you mean by a 'decoder'). Any satellite receiver, without a viewing card in the slot, will get 'Free to Air' (FTA) channels. The signals aren't encrypted and don't require a subscription. Assuming that the satellite dish is pointing at one of the Sky satellites, you can see a list of the FTA channels he'd receive by looking at those labelled 'FTA' here:
http://www.wickonline.com/fta.htm
As well as 'Free to Air' channels, there are also 'Free to View' (FTV) ones as well. These channels are encrypted but don't require a subscription. The FTV channels are collectively known as 'FreeSat'. You need a viewing card which costs �20 from Sky. (That's a one-off fee. As stated, there's no subscription). Use the scroll bar, here, to see what FreeSat has to offer:
http://www.freesatfromsky.co.uk/?pID=3
Lastly, there are subscription channels, which cost an arm and a leg, from Sky.
There are also 'D2Mac' encrypted transmissions (with mainly foreign programmes or 'adult' content) available from other satellites. However, it's a specialist field and many of the viewing cards are illegally pirated.
Chris