Best way to inform mom with dementia that one of her cats has passed?

Another vote for not to mention it --- or the other living cats -- at all unless she asks.

Don’t tell her.

Don’t tell her .
You can have the cat cremated in a group cremation with other pets it’s cheaper in a group . Check with the vet .

Or if you want to be more eco friendly , aquamation is about $300 for a pet under 30 lbs , which is more expensive than cremation by fire .

Don't tell mom.
Tell the neighbor to burry the cat. (or whatever is legal in your area)
gotta bring this up.
If mom is in a facility (Memory Care or Skilled Nursing) you might want to begin the process of removing any other animals she has and either contacting a shelter or ask the neighbor if they will take them.
Then you have to begin clearing out the house so it can be sold to pay for her care. (if no one else is living there)

You don't and if she asks about the cats (she might not at this point), you tell her all is well.

She doesn't have the ability to process information logically, sensibly, realistically. Trying to explain anything to a person with dementia will only increase their anxiety and confusion and distress her.

The news about the cat will upset her for no reason, at this point.

Tell the neighbor to take care of the kitty and make arrangements to rehome the rest and start settling moms house.

I wouldn't mention it unless she brings it up. Why increase the pain she's feeling at being away from her home and her pet? On the other hand, she might not be as upset as you think she'd be. Memories fade.

Depending on how things go, you could always have sweet kitty cremated. There are memorials that can be made out of pets' ashes, even jewelry mom could wear. I don't think the jewelry is such a great idea in a dementia patient's case because it could get eaten or thrown out when she doesn't recall what it is, but I'm mentioning it anyway because it is available.

I wish you luck in dealing with this!

Don't.

I tend to agree with most of Alva's post. If Mom doesn't ask about the cat, I'd let it go. If she does, say it wandered off-a thing cats are known to do. I'm hesitant about telling mom the cat is dead. It may be a very rough truth for her to handle and might make her agitated-especially since she has dementia. There are life-like stuffed toy animals that are designed to mimic cats. This could be her replacement kitty,

Just wanted to thank everyone for the thoughtful replies. I did not mention him and I don't think she remembers. Sadly. I buried him in the yard with a toy. He was a sweet kitty.

The cat's body will go to your local Animal Control for disposal. If you cannot locate them call the area's police or sheriff offices.
The best way to tell someone with dementia the truth is to tell them the truth.
A life long lived generally recognizes death.
There will, of course, be mourning, which is normal.
It is also, in many areas, not against the law to dig a hole in the backyard to bury the cat, should the elder wish to have this done.