Mother, 95, with dementia, highly anxious she must talk to my deceased father on the phone to tell him she is coming/going to Montana where we used to live. We live in Washington now.
Usually I can put her off by claiming I called him already, or that he's at work, or asleep or at a Masons meeting, etc.
Sometimes she thinks I'm lying and/or insists she wants to talk to him herself.
You'll be back where you started.
This will pass.
Even long term loops always do.
I would think your Mom has been told, at one time, that Dad is gone, she just does not remember. And if you tell her now, she won't remember tomorrow. Some go thru the whole griefing process again. You can tell her again, but don't be surprised when she wants to talk with him tomorrow.
How does one thing that a right to know and to have grief is taken when everything else has already be stripped away?
Just my humble opinion. You must do as you think best for the person you love.