UK: I think my neighbor is abusing an elderly person. How can I help?

My neighbor is a carer for an elderly, distant relative. She goes to her house every day for about 2 hours. The relative is about 80 years old, and is showing clear signs of dementia. She doesn't know what year it is, she doesn't know where she lives, she thinks her father is at war, she thinks her husband is at work (he died 11 years ago).

The problem is, I don't think my neighbor is caring for her correctly, or at all really. She keeps telling me how she soils herself all the time, she doesn't eat much, she doesn't wash, at all, but the problem is, because the relative has memory loss, she thinks she's a 30 year old woman and won't receive any help. She often has aggressive out burst towards my neighbor. My neighbor tells me she's rung the doctor lots of times but the relative won't let him in to the house so he leaves without seeing her.

Apparently, her house is filthy, human excrement, rotting food, she's constantly soiled and does not wash. My neighbor says she refused all attempts for her to clean her or tidy the house.

I only know on what my neighbor tells me, I don't even know where this relative lives or anything else about her. The neighbor often brags about jewelry she's taken from her house, some cash and laughs it off. I tell her how horrified I am and she just says that the relative has lost her mind so it makes no difference.

The relative won't accept any help from anyone. I feel disgusted at my neighbor for taking advantage. my neighbor is the only relative that the old woman has left. My neighbor seems reluctant to get other help.

How can I help, I know so little about the relative and my neighbor won't let on much. I don't know where to turn or what to do.