My dad is being a jerk to my mom.

Your mom should call 911 if your father as much as TOUCHES her with any violence. She can then get a restraining order against him and proceed with a divorce, if that's what she wants to do.

You can't really do anything here because it's her marriage. She's been putting up with this behavior for a long time now, so she is the one who has to say ENOUGH and then actually do something about changing the situation.

If you are in the home and witness ANY sort of physical violence, you too can place a call to 911. Let your mom know that you intend to do just that if you see him raise a hand to her at any time.

Keep yourself safe as well.

Good luck!

Financially stuck? Coping with Alzheimer's is awful under the best of circumstances; your Mom is in one of the worst circumstances, as you know. Is there an Alzheimer's Support Group available in your community?
Is there a local women's abuse shelter for counseling? (Ask Police Dept.)
Is there a doctor who can help communicate possibilities for community help to your Mom? I believe all your Mom can do is save herself, no matter how much she has loved or loves your father, because verbally abusive relationships many times escalate to physical abuse. You, too, need to look out for yourself and stay out of the middle of any confrontations.

Not your problem. It's your mom's. Just let her know you're there for her whatever she decides. If their relationship was gone already, your dad's Alzheimer's should have no bearing on your mother's decision.

HoneyBee; If your dad had a change in mental status and went from being a nice guy to being physically abusive, he needs to be investigated by a competent psychiatrist or neurologist.

Many years ago, a friend who is a psychiatrist was acting as a consultant to a big city fire department. A respected chief had suddenly, out of nowhere, started physically and verbally abusing his wife. She, a smart cookie, called his chief who reported this to the chief medical officer of the FD.

My friend interviewed the fire fighter. He said "I talked to him for about 5 minutes, and I knew EXACTLY where the tumor was located".

I still get chills when I tell that story. The surgery was successful and both the career and the marriage were fine.

Whoa, protect yourself first. After 50 years of marriage, my dad entered the early stages of dementia and mom became horribly verbally abusive to him. She refused to care for him, and told me he was now "my responsibility". I saw it through, moved him into a memory care facility, and after almost three years there he died two months ago. Mom was awful through the whole thing and continued her verbal abuse at his care home. Now he's dead and I'm still stuck watching over her care. It's been a nightmare and I am now in counseling/therapy. I suggest staying out of their marital relationship to the greatest extent possible. I had to be a referee for the last two years or so that my dad lived at home and it really took a toll on me. If you wade into these waters, you will be drowning quickly.

Is it possible that this "jerky" behavior was part of a broader picture of cognitive decline BEFORE dad's diagnosis?

Was this a good marriage before the last 3 years?

Has your mother asked for your advice?

If he has been treating her this way long before the diagnosis of Alzheimers, then there are two options. It sounds like your mother has had about enough that she can take and believe me, looking after someone with Alzheimers is very very hard. It only gets worse as time progresses. If you think it’s bad in the early stages, which it is, you would certainly not want to be around in the mid to late stages. He may probably become violent, incontinent of both urine and faeces, lose control of his muscles so become wheelchair/bed bound, require all food to be puréed as he will eventually find it difficult to swallow and a whole lot of other things too numerous to mention. It is not pleasant for him but very unpleasant for a full time carer. It takes over your life, you have no time to call your own, are interrupted night after night so get no sleep and you are exhausted and done.
First option - put him into a home equipped to look after Alzheimers patients.
Second option - file for divorce.
Good luck.

File for divorce? I doubt counseling will work if he has alzheimers. Could also put him in a home.