How to get a caregiver son to take his duties seriously?

She just turned 92. Has been very independent, a vorocious reader, immaculately dressed, able to talk on any subject. In the last three months she has become demented and the son that lives with her has let her become unwashed, she no longer reads, knows anything of what is going on in the news, politics and most of her food she feeds to the dog. I found her looking like a homeless person with her hair hanging down her back and I don't think she knew me. I have known her for over 20 years. I told her son, caregiver, that I needed to teach him how to care for her and bathe her properly and instead he left and was gone most of the time I was there. I got her to the doctor for tests and other then dehydration she was still okay labwork wise so couldn't be hospitalized. Got home health involved but in their area the people who investigate elder neglect are not interested in doing followup as they are too understaffed. The sons are already fighting over her money of course. I live two days drive away and am not a blood relative so have no standing legally. She is left alone many hours a day while her son is at work and the door is left unlocked. She can no longer walk on her own either so cannot get up to get her food or drink while her son is gone to work and doesn't get home until late at night. He will leave her food and something to drink next to her but she forgets it is there and goes with out until he gets home at 9-10 at night. She has very few neighbors where she lives. She has a life alart button but I dont know if she remembers she has it on. After I got home I called for a couple of days and she became very snippy with me and that is also not like her. Her caregiver son left her in the hands of his brother for five days who had not instructions on her care and never told either of them how to reach him or called her and she was most upset that her son was gone. I have had to pull back because her sons don't seem interested in caring for her properly and I live too far away and it kills me to see what she has become. Help!