Is this dementia?

It sounds like Lewy body Dementia. The type of dementia usually accompanies Parkinson's disease. Its main feature is visual hallucinations. Patient sees and sometimes interacts with imaginary people. Another characteristic is its fluctuations (good and bad days).

Since you put this question in the Alzheimer's & dementia category, I'll assume your father has been diagnosed with dementia. Your profile offers no details and neither does your post. The behaviors you describe fit within the dementia parameters as hallucinations (seeing people that aren't there) and speaking to the deceased loved ones, which my mother did all the time when her dementia was advanced. Who's to say she wasn't? Maybe your late mother is speaking to your dad & getting him ready to join her in the afterlife, you just don't know. That's quite a common phenomenon at the end of life for many elders; hospice nurses report about it all the time.

If this is brand new out-of-the-blue behavior and dad does not have dementia/ALZ, then he needs to be seen by his PCP and checked for a UTI or other organic diseases/infections that could be contributing to this behavior.

Good luck!

Or an electrolyte imbalance?

A change in mental status like this is an emergency.

Call dad's doctor right away.

Your profile says your dad is 47. I assume 85+ is the right age?

Does he live with you? How's his short term memory? Ask the same questions over and over? Hallucinations and talking to dead people could certainly be signs of dementia but could be other things as well.

I'd get him to his doctor to talk about these issues and get a cognitive evaluation to see what's going on. Could have a UTI? Could have a vitamin deficiency or a thyroid issue, etc.