I'm so sorry. I spent my career working with people with dementia and the late stages are an especially sad time. If you are able to (and want to take the word of an internet stranger), sometimes heavily sweetening food makes it more appetizing for people with dementia as they lose the ability to taste everything but sweet. A couple of packets of sweet n lo (the pink envelopes; it is the sweetest) added to any food makes it have some flavor again. It is definitely counter-intuitive to do this as sweetening most foods sounds gross but I've seen it help.

I agree with Bizi that this may be a good time for your family to consider hospice.