Somebody has to stop her. It's that easy. And that hard.
Some people have no problem dealing with it -- taking the keys away and letting the poop hit the fan. My husband had a terrible time doing it.
My MIL has alzheimer's too. At that time it was not diagnosed and was pretty early on. Her real issue was all about independence. She loved driving, she claimed she paid for her car, she paid for the insurance, therefore nobody had the right to stop her.
When I caught her driving without a license and took the keys out of her hands, she called the police and told them that I'd stolen the keys to her car and to arrest me. (Mind you, without me she'd be up a creek, and she knows it.) But this was all about a single-minded panic at losing what she considered the last thread of her independence, and her focus narrowed down so tiny that NOTHING mattered to her but that she still be able to drive. Even other people's safety. She didn't even bother to claim nobody would get hurt. She just said she didn't care if she did hit somebody, because she'd probably be dead and wouldn't have to know about it.
And there was absolutely NOTHING we could do to stop her (unless of course my husband had been willing to take her keys and put up with the aftermath). When she had to go to take her test again (after her cardiologist wrote the state and said she shouldn't be driving) she failed the test five times (oh, the stories the testers told me, oy!) but insisted that everybody was lying because they just thought she was too old, because she knew she was a good driver. She decided they were discriminating against her because of her age, and called the DMV repeatedly, until finally I got a call from the Captain who was in charge of the place begging me to stop her from calling -- she was calling him 15-20 times a day to protest that she was a good driver and his testers were lying.
I felt like saying, "If a big guy like you with a gun and state police unit to back you up can't stop her, what am I supposed to do?"
(sigh)