Do I have to tell a parent they've been diagnosed with dementia?

Have the doctor do it.

My dad still tells people that I am an effing liar because I got caught in the trap of telling him to save the doctor the discomfort or whatever his cowardly hide was worried about. Had I known what the fallout is I would never have said a word.

You mentioned she's incapable now. How incapable is she? Does she still possess enough brain function, cognizance, and good judgement? If so, then she must know so that she can plan for her eventuality and put her affairs in order. If she chooses not to do anything about her diagnose, then it's her choice.

If she is too far gone, then she might not remember that she has dementia after you tell her. It may just cause her pain at the moment you tell her. After that, she may just forget.

"Should I wait and have her physician tell her?" Who was she diagnosed by? If it was a dr., he/she should have told your mom. I find it immoral for a dr. to diagnose dementia and not tell the patient. Sadly, some drs don't have guts to tell because they're afraid of the families reaction in the office. They would rather put the onus on the family. No, you should not have to tell her.

No, her doctor should do it. Mom's neurologist sat right down in front of her, looked her in the eye and told her.