In the evenings, my mom usually creates a scene claiming Dad is a stranger in their apartment, ir that she needs to be rescued from him, etc. They are in a senior apartment where you can add "assisted minutes". There are only "aides" there at night. They call family if there is a problem. Second option is call 911. I am the only one to call since my only sibling has lung cancer. I only live 5 minutes away but my days are consumed with meeting the needs of my parents. They both have Altzheimer's with Mom more progressed than Dad. Their greatest fears are being separated. But now my pooe dad is spending his evenings kicked out of his apartment in the halls or lobbies at this facility confused and waiting to be let in to go to bed! I know she can't help what she is doing but what do I do about it? It feels like she is abusing my dad. Meds at night? Move to memory care place? I feel like he is not ready for that. Separate them and give them each their appropriate care? She gets UTIs that seem impossible to keep cleared up. We keep trying. Sometimes I have weeks where I am there 4x/day to give her antibiotics. She also falls alot. Sorry this sounds so scattered but it is a good representation of how my head spins as I am trying to problem solve.
And, I'd explore appropriate care for each of their levels. While your dad may feel quite strongly about them staying together, he may not be sharing just how stressful being with a person who is significantly progressed with dementia can be. With his condition, he can't be relied upon as a caretaker of her, nor a supervisor of her care. It's just too risky for them both. I think I'd seek an honest assessment to determine what level of care they each need. Perhaps, that can be in the same place, but, I would do whatever is required.
I know this is a sad story, but, a family friend of ours believed that there were children inside his house making noise. He left his house to get away from them and fell in the street, fracturing his hip. He went downhill from there and never recovered. I'd take her delusions or hallucinations very seriously. She might hurt herself or others out of fear.
It's just part of the dementia. Offer her possible solutions...like someone to "keep and eye on him" to verify she is safe. I found that to argue with the person in sundown was a fools errand. Smooth it over instead.
I kept my Dad home. He was never in NH or other institution.....granted there were some really rough spots, but the sundown was not the hardest thing to work around.
I'd start by discussing the issue with her doctor and see if anything can be done to minimize her fear at night. And then I'd also start looking for other living arrangements for them.
It really does sound like they don't have a high enough level of care where they are now.
When this happened with my now deceased mother, my sister (psych nurse) advised not to challenge or debate her, but try to address the particular delusion.
When your mother thinks your father is not her husband, play along with it and, e.g., say that he's a good friend and will stay with her until your father comes home. Have him change his shirt before sundowning starts, then after he's been with her and hopefully she calms down, he could say he's going to look for your father. Go someplace like the bathroom and change his shirt to one that she hopefully would recognize. It might help her think that he's actually her husband. If not, well, maybe a higher level of memory care might be appropriate as this must be so stressful for your father.
I'm not normally a supporter of meds, but you could ask her doctor if there's anything more appropriate for her that could help ease her anxiety. It's got to be stressful for her as well.
I think one of the most emotional situations that occurs in dementia is when family doesn't recognize other family members.