Do you want to buy into DVC or do you just want to "rent" your friends pts for a vacation because they don't happen to be going this year.
If you are trying to figure out how to rent his pts this year then you would figure out when you want to go, see how many pts it takes to stay in the particular size villa you want and then rent his pts, he will make the reservation for you in your name and you would negotiate a fair price with him. You can get info on renting on the rent/trade board. Doing it this way usually allows you to stay in a villa for less than it would cost if you booked directly through Disney.
If you want to buy in, ask your friend for info and then check out the DVC forum on this board for more info. You can also get info from Disney Vacation Club and it is pretty well explained and you can buy certain amounts of pts at certain DVC resorts.
Anyway, do you want to purchase all their pts for the rest of their contract or just for this year.
If you are doing an outright purchase, you can get a lot of info on these boards about prices.
I'm sure someone else on these boards knows what you would have to do if you just wanted to take over his contract. I assume he wouldn't have to list with a broker to sell and this would save him money and therefore maybe save you some, too.