
My dad has Alzheimer's. Yesterday, my mom says to me, "I usually know first thing in the morning what kind of day we are going to have." She was referring to the (bad days) bad attitude, frustration, and mood swings or (good days) calm, conversational, and looking out for her good. Is it typical with Alzheimer's Disease that the patient wakes up in a mood and stays in it all day? I am still trying to help my mom accept the fact that this is an actual diagnosis and not "just old age", "too many medications", "low vision", "hearing loss", etc. Some days my mom seems to accept this and other days she is certain he is "getting better".
You could give her some dispassionate facts when it seems appropriate, in reasonable doses. If he starts having delusions and seeing people that aren't there, you could have the facts ready and say "this is a symptom and you should check dad for a UTI or other infections, otherwise it's just part of the disease process. I've learned that you should go along with the delusions as long as they are not harmful".
If she says she knows how the day will go/the mood will remain for the day, I'd just say oh that's good mom. I wouldn't bother arguing over whether or not she is "right". Maybe she is, maybe she isn't. It doesn't really matter. What does matter is that you are prepared for the fact that your dad might change on a dime at some point and fly into a rage that mom was not expecting, or have other objectionable or potentially dangerous behaviors. Then you can present mom with facts and help protect her and your dad.
Just keep in mind, you can't change someone else, only how you react to that person.
Perhaps it's just your mom who allows your dads initial mood to dictate how her days go, without realizing what she's doing.
Might be helpful if you brought her some reading materials about dementia to better educate her so she's better prepared for what the future holds. The book The 36 Hour Day would be a great place to start.
I doubt that you're going to convince mom that his diagnosis is real. She seems to be in a delusion that she's manufactured to help her cope. In fact, I suggest you make sure she's evaluated for cognitive issues that she might have. It is quite possible, given what you've told us.
Dad isn't going to get better. Don't even discuss this with her! Don't argue either, because you can't win. Just steer her to what SHE needs now so that YOU can make the plans you may need to make. It won't be easy, but it is necessary.
Good luck with what you need to do.