Hi there, and welcome to the forums
Has your friend got any other telephone equipment in the house? By this I'm including other phones at other points, sky boxes (they can be particularly bad and normally require quite high-quality filters), alarm systems, etc.
Not sure if you're aware, but all of these have to be filtered too.
Without more information it's obviously difficult to make assumptions and recommendations. What I find useful is sometimes to draw a diagram of your phone wiring and work out at which points it's best to place the microfilters.
Remember of course that the telephone port on a microfilter cuts off the broadband signal, so you can't plug the modem into an extension that has already been filtered further upstream, if that makes sense?
If there's no other equipment plugged into the phone line, it might be worthwhile trying a different model of filter, although the ones BT give out are usually quite good.
Failing that, it might be a fault with either the phone or modem, or even on the telephone line, but from the symptoms described, it sounds quite unlikely.
Hope that helps, please don't hesitate to post back with further information / queries if you need
