How do I convince my mom that Dad isn't coming into their bedroom at night?

I have been with my parents for almost a year now, my husband is still in another state and planning to move in with us soon. Mom only feels safe when I sleep next to her. She thinks Dad is coming in at night doing something to her. I tried explaining that Dad will not come in "her" bedroom, but she doesn't believe me, I know it's the dementia.