I don't see how an ISP could be assumed to be involved - it merely provides a gateway to the internet. However, distributing scam or infected e-mails I do believe should count against ISPs, although it's not generally accepted by them.
I'm presuming she has an internet connection. If she has not taken the proper precautions against receiving such "dialers" then she could just as equally be held responsible.
She should contact click here and make a complaint and also, if NTL does the same as BT, have outgoing premium rate calls barred from her telephone.
The only occasion I have had such an occurance proved very puzzling (it was some years ago), until careful examination of the phone bill revealed the calls had been made at times we were absent, but had a workman in the house....:-)
When tackled with the evidence, he didn't argue, just paid up plus quite a bit more.