To use any wireless function, you need a wireless router.
BT do their own in the form of their BT home hub, or you can buy others yourself if your friend just got the package with a USB modem (no router).
If you have only the USB modem, then you need a wireless ADSL modem+router before you can even think about his wireless laptop. If you need a modem+router, I can recommend this one:
http://www.ebuyer.com/UK/product/118010/rb/0
Once you have that or the BT home hub (something to transmit your wireless internet, basically), you can think about the wireless laptop.
If the laptop is less than about 3 years old it'll likely be wireless already. If it says Centrino somewhere, it's wireless. Otheres, as Ethel says, have a little switch on it saying on and off, with a transmitter type of icon.
If the laptop isn't wireless, you need to buy a USB wireless dongle. The package I link to above includes one, but you could buy another from that site if need be.
(Apologies to Ethel for butting in on her answers.)