Diphenhydramine aka Benadryl is used for allergic reactions, to prevent or treat active motion sickness, and for mild cases of Parkinsonism. The last two uses (motion sickness and Parkinsonism) are based on the anticholinergic effects of diphenhydramine, and not its antihistamine effects. So Benadryl helps Parkinsons.
But Alzheimers medication is just the opposite.
These medications, such as donepezil (Aricept) work by inhibiting the breakdown of acetylcholine, which sends signals in the nervous system. By contrast, anticholinergics—such as diphenhydramine (Benadryl) —block the action of acetylcholine. So if I give mom Benadryl and she gets more confused, is that a hint she has Alzheimer's?
If she has some tremor, kind of a cogwheeling feeling when you move her wrist or ankle, like a therapist doing a range of motion, there is more likely a Parkinson's related thing going on (vascular or Lewy body) and a good neurologist and most geriatricians would pick up on that and maybe do a little Sinemet or something else that might help a little. You probably don't have to put her through a full formal neuropsych eval. I've often had to teach non-neurologist docs how to look for that.