How do i deal with this awful situation

I've spoken to a few friends about this but I need more input on this matter.

I am dating this guy. He's awesome. Been together 10 months now. He lives at home with his parents, his sister, and his grandma (his dad's mom) all in the same house. He had to move back home a year ago after he lost his job and is trying to get back on his feet.

Anyways. I've been over to his house a bunch of times and I'll be honest his mom is pretty grumpy and manipulative and not the easiest person to get along with.

His grandma just turned 90 and she has some form of dementia or possibly alzheimers disease.

(Just a small note! My mom worked with individuals who suffered from dementia, alzheimer, and cognitive issues and I did my high school volunteer hours as well as coop in that department for 2 years. My mom is really educated on the matter and I have a little bit of knowledge too)

I have seen on multiple occasions (within the past month) his mom freak out and yell at this 90 year old woman for such small things such as flushing the toilet twice. Or for forgetting to close the fridge door. Or for barging into rooms without knocking then staring into space and then walking away.

This morning my boyfriend came over and was telling me that this morning his grandma has been waking up between 2am and 6am (including last nigbt) and repeats random words or phrases on and on for hours. And as per my boyfriend (I was NOT a witness) his mom took a spray bottle of water and sprayed it into his grandma's face and told her to shut up, swore at her, yelled etc. And his grandma was obviously stunned and cried "What did I do? What did I do?"

I was like you need to call the cops. His mom is always abusing her. And the thing is they know she has dementia but wont seek out extra help and don't want to put her in a home. Why? His mom's reason is so they can take half of her pension as rent. His mom just pockets the money and his dad has no idea that she's taking money from his mom. (Please note it is only his mom who is abusive to her. His dad who is her son is never around when she responds to her like this)

The dad has no clue because he works so much. And he feels like his mom did so much for him so he will keep her home with him not in a long term care home.

I told my boyfriend that they need to address her state and get her the help she needs as well as report his mom for elder abuse.

My boyfriend in turn flipped out on me calling me a know it all who insists that his grandma is fine mentally and that I don't know what I'm talking about.

What do i do?!?!