How do you explain to an elder with dementia that someone has passed away?

You may want to read this article
https://www.agingcare.com/133806/
Explaining death to someone with dementia is difficult, and then they forget and you have to tell them all over again - or come up with something to say. It's horribly hard.
My personal feelings (explained more in the above article) are that you tell them the truth, as they deserve that right. Then, if they keep asking, you can say things like "You'll see him soon" or whatever works. But everyone must do what you feel is right.
My heart goes out to you. This is so very, very hard.
Take care,
Carol