Why is she paying for broadband she doesn't use? If she were telephone only she would be migrated to 'Pre-Digital Phone Line' (Home Phone Standard) where the switchover is done in the exchange and nothing changes in the home, extensions still work and no risk of electrical power outage affecting the phone line. Given her vulnerable status it might be worth asking BT if they'd be willing to migrate her to PDPL. They probably won't, as having broadband precludes this, but you never know.
