Um, Im not quite sure what you mean. Are you saying that your mum doesnt remember that your dad has died and thinks that he has left her instead? This happened to mum - she was sure that her husband (died 30 years previously) had left her for another woman. She could not remember that he had died so her brain manufactured this "reason" for his absence. Are you constantly trying to explain to your mum that her husband has died? If she has forgotten, then everytime you tell her this will be like the first time that she has heard.
Mum also imagined another husband (who was constantly late home!) who would sort everything out for her. Im sure this was due to anxiety as once she moved to a care home this imaginary person disappeared, although soon after the move mum thought that one of the other residents was her husband! She seemed to need the comfort that there was someone who would look after her.
With mum, we could not let her believe that she was married to another resident, but your mums delusion is not harming another person, so I would not try and convince her of the truth - she will not remember what you have told her, but will be left with the emotional memory that there is something terrible about her husband, so she keeps asking about him. I would just go along with it and say that her husband is working late or something like that. If her husband used to have to go abroad for work then this is an ideal excuse.
She may also need something to help with anxiety