If I may take a philosophical bent, how can you be sure it can't be that your husband believes he sees his father?
The medical profession will label your
husband seeing his father as a hallucination.
Do we really know what is possible in our universe?
What if people with dementia have a special ability to "travel" through multi-layers of time in space to connect with those beyond or geographically elsewhere? I have seen enough examples of people with dementia connecting with loved ones on the "other side" that I can no longer be certain what really exists.
A safe approach is to remain by his side, learn from his experience while preserving his dignity.
Take the lead from your husband and ask him what his father is doing? What is he saying? Allow him to talk it through. Ask him what scares him about the experience.
Oftentimes, instead of rushing to judgment about what is or isn't, we can give the gift of comfort when we simply sit beside our loved one and go along for the ride. Who knows what you may discover in the process.
For more information about "hallucinations" and Alzheimer's, click on these Caring.com articles below.
Alzheimer's Symptom: Hallucinates (sees, hears, smells things that aren't real)
Is it common to have hallucinations with Alzheimer's?
How can I handle my mother when she has hallucinations while saying she wants to "go home"?