I would hesitate to tell her by using either the names of alzheimers or dementia. At least when it comes to dementia, one of the first symptoms noticeable after some memory loss is "distrust" developed.
You should absolutely bring her into a doctor for a diagnosis ... and maybe you could use some reason other than an implication of a memory disease.
My Mother recently passed from Dementia three months ago. Her Mother over 20-years ago had passed from complications of alzheimers. My Mother had a tremendous fear of that disease watching her Mother die after years of continuing downturns.
I hinted at her having Dementia twice - long after it was diagnosed by a doctor and she was in a home for dementia and parkinsons! Of course, she refused to believe it. Then, I said no more about it. What good would it have done for her to believe she has what she was so afraid of.
I have been told by nurses at her home - and later by the nurse from hospice, that Namenda does work well in the earlier stages but not that well in the later stage of dementia.
Please seek a doctor ... try to put together a question list ... and remember that patients are differ to some extent, so don't generalize too much.
Peace and wellness
Rob & Gizmo