My mother is having hallucinations and is convinced my father has other women in the house at night. How do we convince her it's not real?

Some of her meds could be causing her confused thinking. Talk to the doc.

Get her to a medical professional as quickly as you can. There are medications to help with hallucinations. You will not be able to convince her until her frame of mind is corrected. If you can't get her into medical care due to Covid please call her primary care physician and they can better assist.