I would try the basics first - are the leads still in securely (PC, modem and filter). Try a different filter (cheap). Check the lights on the router. If you have no spare to try, then probably best to contact ISP next (yes, I know that can take time and money) as they can check to see if the problem is with their or your friend's equipment or BT. BT won't action broadband faults without the say so of the ISP anyway. If ISP says it's likely to be a modem fault, then at least you will know!
Using this
http://www.samknows.com/broadband/checker2.php will tell you if a BB service is still enabled on that phone line