What can help 91 year old with hallucinations that scare her?

I'd report any change of mental status to her doctor and make sure the doctor knows what's happening. I might ask for a referral to a geriatric psychiatrist, if her primary can't figure it out and help.

Dementia causes these hallucinations. If they scare her, there are meds for this.

When you say lab work are you talking about urine test for a UTI? Or blood work? What meds is she on? Is she dehydrated? That would show up on blood work or possibly by pinching the skin.