Living with hallucinations from two sides
I care for someone with Dementia who had hallucinations while in hospital and I live with my own hallucinations from Lewy-Bodies.
My BIL saw large black slugs crawling everywhere so I would tell him to pull the sheet over his head so the spray didn't get in his face. I'd then make a hissing sound and tell him the slugs had been killed. This stopped them being a worry. The kitten under the bed I told him I'd take it home with me when I left. I never argued what he saw.
For my own hallucinations, sometimes I recognise them for what they are, my late abusive biological mother, a Roman legionaire who scowls at me, a little girl who does handstands on the shelves that hold work in the pottery class I attend. These I just keep reminding myself are not part of my today.
The other more horrific ones I have real problems with. I've taught the folk in my life not to to deny what I see, hear or smell because it doesn't help me one jot.
They are real for me in that time frame and no amount of denial/argument changes it, just adds more stress.
My suggestion from living both sides with hallucinations is to meet the dilusionall person where they are and not where you would like them to be.
If the hallucination causes distress try to deal with it, if not then go with it.
Sometimes both I and my BIL can recognise later that we were hallucinating, if it's him I say "yes M, you were seeing ...... but that's ok because it shows your imagination is still in full working order." Agreement from my people that I was seeing things helps, then I know for a little while longer what is happening to/with me.
Hope this helps