If you're working with someone who has Alzheimer's or another disease which causes dementia, you need to remember this one piece of information. The other person's reality is NOT the same as yours.
I cannot stress this enough. You are not dealing with someone who plays by the same social rules as you. The internal governor is broken. The person cannot process their experiences or their emotions in the same way you can. Heck, with the progressive nature of the disease, they may not be able to process in the way that they could have a year ago.
So, if you're caring for someone with Alzheimer's and something hurtful is said, you MUST consider the source.
It is often the disease talking.