After 70 you have to renew your licence every 3 years (i think) but I'm not sure if you have to provide a medical certificate. In any case you are required by law to notify the DVLA of medical conditions and there is a minimum standard of vision. I imagine she is reluctant to give up her independance but from what you have described i doubt she'd pass a medical. There is nothing really stop her getting in a car and hoping no one finds out. If she did have an accident I think the state of her health, if she's not informed them of it, would invalidate the insurance. You are obviously not comfortable with the thought of her getting behind the wheel so I would try to discourage her or at least get her to go to the doctors for a medical first.