Did she use a web browser or a mail app? If a web browser then that is webmail and would register as being used, if a mail client(or app) you need to check the settings in the client/app to see which server it is using.
Is your mum a BT broadband customer?
How long ago did she stop her BT Broadband account, or is she still paying for it each month?
If not, did she move her account over to BT Premium mail, which costs £7.50 a month within a short time?
She has been told by BT that the account has been deleted and cannot be recovered.
This is why I feel that the only option is to make a formal complaint.
I am looking please for the contact details/ process to be able to do so?