How do I tell my late Dad's physician that his dementia diagnosis' was half-baked, and Dad was more with it than he thought?

i think your brother will lose unless your dad was declared incompetent . a quick competency test might be as simple as asking the patient where he currently was , whos the president , etc .
dementia is a medical diagnoses , but that dx doesnt stop an elder from managing their own business ..

Were there other doctors who saw your father during and after that time who didn't concur with this physician's DX?

Have you contacted the attorney who made the changes to determine if there was any indication during the signing that your father wasn't capable of understanding what he was doing?

Exactly what do the doctor's notes say? That Dad had dementia? That in itself would not disqualify him from changing his will. That Dad was incompetent to manage his own affairs? Did the doctor administer any tests?

Are the changes to the estate plan outrageous? Did he change from leaving his estate to his relatives to dropping dollar bills out of a helicopter during a state fair? Are the changes in nature of things Dad would never have done "in his right mind"?

I hope you have a lawyer to represent your interests.

Rereading your post title and your post, it's not clear to me why you would want to even bother PERSONALLY challenging the doctor's DX, since you say your brother will be using it as evidence in the will challenge. Most likely, the doctor would refuse to speak to you since he's been contacted by your brother who's contesting the will.

I don't really see any point in tell a doctor his DX was half-baked; I'd focus on written documentation to contest the will challenge.

There were other doctors that saw my father during the same period and they didn't note dementia in their reports. My father's attorney questioned my father behind closed doors and was confident that he knew what he was doing, but my brother is using the physician's notes as the basis for his capacity/undue influence challenge. The changes were only outrageous to my sibling because in his mind, the inheritance was set in stone never subject to reconsideration. The reason for the changes to the estate plan stem out of my brother already receiving substantial sums of money over his lifetime, that factored with the selfish behavior exhibited following our mother's death most likely compelled him to reduce his inheritance from 50% to 25%. The reason I say the diagnosis was half-baked wasn't to insult the doctor, but to distinguish between a definitive evaluation and one that came about during a visit that lasted only minutes. I suppose the doctor will have to give details on how he came to his conclusion. Thanks for all of your thoughts and suggestions, they are very helpful.

Be sure to share these facts with the attorney who's handling the challenge for you.

It wouldn't hurt to document, if you have access to the financial records, the substantial sums of money already given to your brother.

Same with the behavior after your mother's death.

All of this will help build the case that your brother expects a given amount, when in reality the inheritance decisions are those of your father. A mistake I think a lot of adult children make is assuming that they're entitled to something regardless of help and/or behavior for their parents.

The fact that the doctor only saw your father for a short time will diminish the efficacy of his conclusions, especially if the other regularly treating doctors didn't make the same conclusion.

I want to thank you also for sharing this personal aspect of your family's life as it's given me some thoughts on what I may eventually have to deal with as well.

JB cant stop laughing but have to pee now! "Are the changes to the estate plan outrageous? Did he change from leaving his estate to his relatives to dropping dollar bills out of a helicopter during a state fair? Are the changes in nature of things Dad would never have done "in his right mind"?
I know this is possible but its just so funny the image of him in that helicopter!!

The only beneficiaries of the contest will be the lawyers. Your brother knows he can't win, he's just going to make it drag on for years and cost the estate thousands in legal fees. It's all about spite, all about making you upset and so far he got what he wants. Do not contact the doctor, your action would be interpreted as attempting to tamper with a witness. That in itself could cost you.