My mother is suffering bouts of dizziness and has been hospitalized twice for brief periods. She insists she is still fit to drive. We don't think she is and think she may be unfit under the car insurance.
This is a common problem with many drivers of advancing age. It's really impossible for anyone other than your mother's doctors to determine her ability to get behind the wheel. Talk to them. They might sign a form stating she is not medically allowed to drive. Ultimately, it is up to them.
Good luck and don't hesitate to ask if any further questions arise.