Are you sure it was sky that got him to take out the warranty.
Several companies phone people up on the off chance 'i'm phoning about your sky box'. They didn't even know who I was. At that time I had only ever had cable.
Those companies have been on watchdog and other programmes before. Despite skys aggressive sales push to get you to go with their services, surprisingly they don't really push the extended warranty thing nearly as much.
Check your dads documentation closely and check the address and telephone number up. If it turns out to be the aforementioned, makes sure the it is not on a recurring deduction every couple of years.
And if you speak to thi company don't believe any nonsense that sky sold his details to them.