How do I deal with my husband's hallucination that I am cheating on him with the neighbor who is INSIDE our house?

Dear lorijo
I’ll confess, since I don’t deal with Alzheimer’s I haven’t seen all the Teepa Snow videos but I think I would check them out for guidance. I know generally she suggests that you go along with and then divert. That you don’t try to argue or convince him to change his mind. 
“ I know it upsets you for him to be here in the house. I’m upset too.”  and then “ would you like to ride to the soda shop? I feel like a chocolate shake, what about you?”
Now this sounds nonsensical but so are his accusations and if it works then use it. 
It’s certainly crazy making to live with this situation. I’m so sorry. 
The best news I can give you is also the worst. The disease will progress. He will stop this behavior after awhile and there will be something else. Again, I’m very sorry. 
Hopefully others will have better suggestions. 

My dad's delusions began innocently enough. He thought there was some kind of shadowy government agency after him but because he was aware of it (in his mind) he had the upper hand and wasn't concerned by it. Eventually this agency became a threatening presence in my dad's life and he became scared. I never tried to convince him that it wasn't real because to him it was very real. Instead I comforted him and reiterated to him that nothing bad would happen to him and that I would always protect him. This worked with my dad. 

I'm not sure it's a good idea to admit that the neighbor is actually in the house but I understand the point the poster was trying to make. Instead of agreeing that the neighbor is in the house you can tell your husband that because you keep the house locked up tighter than a drum no one is getting in without your permission. And then divert. Delusions change and evolve and while this neighbor may be having an affair with you (in your husband's mind) the neighbor could become threatening at some point (in your husband's mind). You don't want to concede to your husband that the neighbor is actually in the house.

Lori, if this is new, i.e., a sudden change in mental status, I'd get in touch with his doctor. He might have a UTI, which sometimes causes delusions in elders.

Delusions, obsessions and paranoia are all part of Alzheimer’s/dementia. But knowing that doesn’t make it any easier to handle. When my mother first started voicing her delusions, I tried to convince her they weren’t true. I spoke with the Director of Nursing at her facility and he told me to develop “Teflon” skin.

Arguing with your husband and denying his accusations will not work. As Eyerishlass says, use diversionary tactics. That’s what I did with my mom. We’d go for a walk down the hallway to the lounge. I’d tell her stories about her Great-grandkids. I didn’t deny her delusions but I didn’t agree with her either. Sometimes I’d just give her a blank look and say, “Oh, really?”

There is a book written by Xavier Amador titled "I'm Not Sick and I Don't Need Help". It is about dealing with schizophrenia but there are some very helpful tips for how to talk to someone who is absolutely convinced that what they see or hear or think is based in reality. You may find it helpful.

Talk to you doctor about this, I knew a man who got violent with his wife because of this delusion.

I dealt with the exact problem.
Talk to your doctor there are medications that can
help with this problem.
My husband is on resperidone .Very small dosage .
If I try to ween him off of the meds he starts again . For me I just keep
him on it.
Good luck. Eventual this part of the disease will pass.

This must be so hard and sad. Do you have others coming in for visits sometimes, females but also males he’s known for a long time and trusts? To ‘people’ his head more and get his attention off you a bit. I pray you get out sometimes, you need and deserve other human connections to help balance this out. Your love and commitment to him does not mean you fight this battle alone. Praying for you both. Hugs.

Definitely check for UTI -- or even just treat for one.

Another possibility, especially with dementia -- is he not recognizing himself in the mirror? Sometimes people see the mirror and (not seeing themselves) ask 'who is that old man? what's he doing here?' It's a short step from that to deciding they know 'who' and 'what' -- especially if a mirror has shown the two of you together.

If you cover the mirrors, this delusion may go away.

This is such hard stuff. I'm so sorry you're having to deal with this.

Good luck!

You start by calling his physician as one of his medications can be causing the hallucinations.