Is it that your mum is using BTWifi from one of her neighbours BTWifi signal? If that is the case the neighbour can opt out at any time and that would stop others from using it.
It could also just be that their BT Hub is needing restarted. BT can not do this but if she knows whose BTWifi she was usng she could ask them to restart their hub. They of course are under no obligation to do that so the may decline to do it.