Any advice on dealing with a parent who is controlling access to the other parent who is in dementia decline?

If you think that she is not safe and he is not providing proper care you could try to obtain Guardianship. This would not be a pleasant thing to your dad through as well as other members of the family.
Adult Protection Services would be another option.
If you have a local Senior Center you could check there to see if there is a Social Worker that you could talk to about the situation.

I am sure he is doing this to protect her and prevent her from being embarrassed by her decline.

My father did the same with my mum who is also in dementia decline. However he didn't do it out of duty to her, but for strictly selfish reasons. She was still capable of serving him and caring for him, just no short term memory, so he wanted her around to help him.
We had a social worker regularly ring to try to reason with him, but he was so irascible and would listen to no one.
The only thing that broke the status quo was my reporting him to DMV and having him lose the ability to drive.
Does your father still drive?

"blocks access to her by friends and family". I am sorry you are going through this and trying to rationalize his behavior, but....his behavior is not rational. I agree with 97yroldmom. I think you need some outside intervention whether it be council on aging or social services. Please tell us "someone" other than him is allowed to see her..otherwise...seek help even though it will anger him. doing nothing might be something you live to regret. Good luck.

Do you have any reason to believe she is in harms way?

That her care is not adequate? That she would like her life to be different?

If you don’t think she has proper care you could contact Area Agency on Aging and ask for a needs assessment for both of them.
You could also speak with APS.

I know how frustrating this must be for all who love your parents.

Its like he wants to capture “time in a bottle”.

Of course one could file for guardianship which is expensive and time consuming and might not happen in time to see her depending on her condition.

I suspect your father has a degree of dementia of his own although perhaps not to the degree that he would be deemed incompetent.

Tell us more about their day to day life. Does father drive? Does he shop? Is there a caretaker or housekeeper? Is anyone allowed to see her?

Sometimes there are family members who are inadvertently enabling this behavior by propping up fathers illusion of independence.

It’s easy to do this as it’s so gradual.

Tell us more. You seem to have a grasp on dad’s motivations but how is he pulling it off?