Get her to call three and say she wants the contact details of the
telephone regulator to lodge a complaint with them. They might get
scared at that point, but probably not. She will then have to write to
the regulator stating her case etc. A few weeks later they will get a
letter from the regulator and at that point will probably drop it.

I had a phone stolen from Vodafone and someone wracked up a £700 bill
overnight calling India despite my having a £100 a month call limit. I
refused to pay and after a few letters from them and a few letter from
me it they left it. I never got around to writing to the regulator and
they never got around to fixing a court date.

I am fairly sure that they will actually have to get a judgment in
court against her to send in the collectors. It's not like parking
offences where you are automatically guilty and it can quickly result
in your car being towed away from your house and sold at auction. Read
the letters very carefully, don't panic into paying the claimed
escalating bill - it's all bluster designed to make one cave in.