While I can change the subject with my mom, other patients there tell the truth.
When my 94 year old mother says she's going home, I can change the subject or just tell her I don't know yet but some of the patients at the nursing home will tell her that they are going to die there, they are not getting out ever. What do you do? I am not there all the time so I can't keep her from talking to people. She has dementia and does not remember a lot of things but sometimes when I go to visit she is sure she is going home that day and maybe it is coming from other residents since I heard one telling her that she would die there and told an other patient that she wasn't going home either.
There is no way to keep your mother from talking to these other people without putting her in solitary confinement. The seniors I know, with dementia, in nursing homes are not stupid - they know full well that they will die there.
People who are depressed will often say things that others find upsetting. My sweet natured aunt kind of lives in her own little world, and tries to ignore the negative people. Hopefully your mom can do the same.