As a POA for 90+ year old parents in Nevada, what can be done when the father is verbally and sometimes physically abusive to the mother?

He already did something harmful.

You certainly can and should do something about this abuse. Tell him if he treats her that way one more time, you will call the police. He will do it again. Then you call the police and insist he be removed from the home. A lawyer gave me this advice long ago.

If you knowingly allow this domestic violence to continue, you are complicit in a CRIME. It is emotional abuse, physical abuse, elder abuse and who knows what else. She is in danger.

The police will sort out where he needs to be after you file a complaint. Get the caregivers to record on their cell phones (video, audio) what is happening.

Check out https://www.thehotline.org where they have live chat with free counselors. Or call them at 1.800.799.7233 All communications are confidential.

"We cannot put him into Memory care unless he does something harmful."

I do not understand this. Memory Cares will not care for agressive or someone who could cause bodily harm to another. They have other residents to consider and not set up for this type of resident. If the husband is physically abusive call the police and have him removed. He needs help and should be placed on a facility where he gets medicated for this problem. I would refuse to have him back in the home because Mom is afraid of him and is not able to defend herself. MC maybe a possibility if the meds work.

I so hope parents assets have been split. If not, POA needs to see an elder lawyer. Dads split would go to his care, when gone, Medicaid can be applied for. Mom's split goes towards her care. She can remain in her home. More to this, but elder lawyer can explain.

You are POA (your partner is)
Your partner's responsibility is to keep BOTH safe. Even if that means safe from each other.
Since there are caregivers there 24/7 if they hear or see ANY signs of abuse they are to call 911.
If he has been diagnosed with dementia the person that has POA can make the decision to place that person in Memory Care.
And I have to ask....you say you can not place him in MC unless he does something "harmful" .... Isn't physical abuse "harmful"? or do you wait until he pushes her down the stairs, or chokes her, or puts a kitchen knife to her throat? (each one of these happened to a friend of mine before her husband was placed in MC)
One option might be to place them in separate parts of the house.

Your profile says “THEY have have demanded to stay at their home”. Your partner’s mother has lived with this for a very long time. Are you quite sure that she would want to be separated from her husband and put into a facility? Clearly both of them together in AL didn’t work, why do you think SHE really wants to leave him? Perhaps it's your partner's idea of what would be best, but that doesn't mean it will work.

Is mom agreeable to Assisted Living?

Set her up there. Leave Dad at home.