It may not help you immediately, but when we had a similar problem with my mum (younger, but had dementia) she would not listen to any of the family. Had she lived, she would be the same age as OP's mum and I think for that generation of women, the freedom represented by driving was a big factor in their lives.
We knew how my mum felt about stopping driving, and we got round it by suggesting that she had a driving assessment. Someone has suggested the IAM, we simply asked the mechanic who looked after her car, who she had known for abut 50 years since he was an apprentice.
She agreed to abide by his decision, and when out with him, did something quite dangerous, and somehow, with him, was much better able to face up to facts.