Hi, I'd like to help you with your question.
It sounds like your mother needs a full physical. For the elderly, there can be many reasons for hallucinations. The most common problem is dementia/Alzheimer's. People your mother's age often experience a decrease in mental abilities, especially when they lose a loved one or people in their social circle. The lack of interaction and social activities encourages loss of brain function.
Another possible cause is a brain disorder, such as a tumor. But for her to have a physical issue such as a tumor, it would have to be slow growing and she would also have other symptoms by now. But it could be a possibility so I need to mention it.
There is also a disorder of the eyes called Charles Bonnet syndrome. It occurs with severe vision problems and it often causes people to see things that are not there. It is mostly seen in the elderly.
If she has never had these symptoms before, I would probably rule out Schizophrenia. She is a bit too old for the diagnosis which is usually diagnosed by age 40.
It sounds like she may be depressed, at least. Though depression does not cause hallucinations like this unless it is severe, the loss of your father could have affected her through grieving that she was never able to completely resolve. She would benefit from counseling if she would be willing to go.
Since she is refusing all treatment, you may want to try your local Department of Aging. They should be able to provide some type of home based evaluation to determine if she is alright to be on her own. They can also provide companions to be with her and engage her in conversation. They may also have many other services that would help her. I am not sure where you live, but if you contact your local city or county government, they should be able to provide you with the information.
You can also contact your local United Way. They can let you know the services available for your mother and may be able to suggest ways you can have services come to her, rather than you trying to get her to go.
Also, consider contacting your mother's doctor. He/she may not be able to share information with you, but you could let them know what is going on with her and ask for suggestions and help, if possible. If you do get her to go to see her doctor, try to get a release of information signed by her so you can talk to the doctor.
I hope this has helped you,