My mother (87) keeps accusing our neighbor of stealing her avocado trees.

She planted some avocado's in a pot. She said that he came and took her pot. The next day she said he came and stole the avocados but not the pot. She went as far as going to his house to confront him. My daughter stopped her and told her to come back home. All this happened when I thought she was in the bathroom. We now lock all the doors and hopefully this doesn't happen again.
When I asked her, how do you know he took them she gets upset with me. This is not the first time she has accused him. She also accused my granddaughter of stealing her radio.
How do I handle her? Should I go and tell my neighbor what's going on just incase she finds a way to his house?