I can see how frightning this could be. It is going to depend on what the cause of her hallucinations are. Is she seeing herself in the mirror or shadows? Is she on any new medications? Does she take any mind altering medications? Is she catching a glimpse of something and is this being construed as a person?
In general do not fight her when she is aggresive or hallucinating. You need to step back and give her a break from whatever you where doing. Also I would recommend seeing her neurologist to see if there could be a cause or a treatment like haldol that would be right for her. Also as a side note do not feel like she is responding to you in anyway. Family is always the people that patients with dementia and Alzheimer"a fight the most.
It may be helpful to get a type of caregiver so that your dad can get a break. Caretaking is exhausting.