Agree with Nitram, the run under a bus scenario/sudden incapacity is a good one, as it doesn't need to address the fact she is losing her memory, which is an emotionally sensitive thing to raise. Point out that if she doesn't do it and ends up in hospital no one can pay her bills, get cash for her etc.
Whether to describe it as helping her depends on her personailty - my mother couldn't abide the idea of someone else looking after her finances. Or another way, if she loves you dearly, is to explain how hard it would be for you and she would be doing you a favour if she would help you out by doing it and otherwise you could end up having to go to the Court of Protection - lots of hassle for you and takes chunks out of her money.
I would also say that you can get the forms, and fill them in, if necessary, all she needs to do is have a long-standing friend or neighbour to act as the personal reference thingummy and witness to signatures. Showing her the forms in my opinion makes it less daunting and doing it yourself feels less formal than involving a solicitor in my opinion and is cheaper. Also agree that doing it yourself, with her, seems a good way to encourage her.
With my own mother I am afraid it was very much a last minute thing which an Admiral nurse very correctly pointed out needed doing as soon as possible. good luck with it, it is such a good thing to have.
