Do I lie to her or try to believe something that is unbelievable?

My mother thinks a neighbor is terrorizing her. I've taken her for neuropsych testing and they just are inconclusive on what is happening to her but are considering dementia with Lewy bodies. My mom thinks the neighbor is climbing on our roof, involving his wife and parent in damaging our property, thinks he has listening devices planted around the house and spends hours a day in our yard, turning on outdoor faucets, damaging bushes, drilling on my house. She has called the police over 15 times in the last year and a half, they haven't found anything and have threatened to put her on pysch hold.

I don't believe anything is happening with the neighbor because I haven't seen or heard anything. My mother feels betrayed that no one believes her and is truly frightened. How do I handle this? I currently refuse to discuss him at all because I have spent hours and days talking about it with her and there is no answer.