Does she have any family she trusts to set up an online presence with their email address in order to set up the direct debit? That's what I do with my elderly dad. Everything he has online is controlled by me and I just do what he asks me to do.
Or maybe she change supplier that allows pre-payment without using the internet? It's a massive problem unfortunately. My father in law had terrible trouble banking with Santander because he had no online access and couldn't use a phone. They expected a terminally ill man to be wheeled into a branch on a hospital bed to do his banking. I kid you not. That was their solution.