We suspect our father is being manipulated, neglected and taken advantage of emotionally and financially by his girlfriend who has his medical POA. How do we report elder abuse?

We suspect our father is being manipulated, neglected and taken advantage of emotionally and financially by his girlfriend who has his medical POA. Right now she is working to dismantle estate planning he's had in place for almost 30 years. She also has control of his checkbook. Besides one trip to a neurologist over a year ago, he has not seen a doctor in regards to his alzheimer's/dementia.
I would like to know if anyone has had experience reporting elder abuse-- what does the process of reporting elder abuse entail? How much documentation does one need? It's been almost impossible to document or observe this abuse as his girlfriend has totally isolated him from friends and family but we have all seen it in snippets and many friends are alarmed, too.
Thank you.