My father has early onset Alzheimer's. I went to one of his appointments a while back, and the doctor said that he was concerned with his driving because the disease makes it difficult for him to multi task. There are too many things that are going on, and it makes it difficult for them.
My aunt and dad both had to get into an accident before they stopped driving for good. Thankfully, my aunt just hit a parked car and my dad just hit a building. They both didn't do much damage, but boy they were wake up calls.
My grandma's brother just passed away last week when he was a passenger with his wife, who has Alzheimer's. She always wanted to drive.
I would say talk about it. Bring up your concern. My mom never told my Dad to stop driving because she didn't want to take away that freedom from him, but after the accident she was forced to and my dad agreed. While the doctor said he didn't want him driving, my mom still let him.
That is all I have to offer. I hope it helps.