You could hire a telephone engineer to do it for her instead.
Or if she is staying with BT for her telephone/digital voice then the digital voice adapters can be used anywhere there is a power socket and a DECT signal to the hub. They will provide a telephone socket anywhere she needs one around the home.
But home wiring has always been the responsibility of the customer. With only a master socket nothing would change, the device would simply plug into the existing socket instead of the current phone.
I don't know for sure, I think that they would provide a device with such a capability for people with landlines only.
There have to be a lot of people who have their telephones plugged into extensions right now. I don't think they will be forgotten.
Or if convincing her does not work, you could just tell her it's a 3rd party contractor BT sent around to help her get her new service in order 😛
She would never need know that the enginner was paid for by you. If you don't mind a little white lie of course!
Yep, the digital voice adapters will work anywhere around the home where she needs to plug a phone into the telephone extension. Just pair it to the smarthub and off you go.
This is all assuming she is with BT for her landline, those adapters do not work with other providers sadly.
Here's a guide for them: https://www.bt.com/help/user-guides/phones/digital-voice/digital-voice-adapter