You need a wired ADSL router and long (10m+) RJ45 cable.
Your friend will need a network card in his machine if it doesn't already have one as the USB connection won't work with a router.
I like Netgear kit myself but there are plenty to choose from; click here
Don't go to PCWorld etc for cables; click here
You want a `patch` cable from the above link, not a crossover.
It is also possible to use Internet Connection Sharing from your friend's PC which won't involve a router but your friend may object to having two cables hanging from the PC (one for incoming internet, one for sharing the connection with your machine) click here
A call to BT with the relevant personal details will enable your friend to find out the password.
At a pinch, running this click here and looking under `Secrets` may reveal it