What can be done when your loved one with mild to moderate cognitive decline accuses a family member of inappropriate behavior?

My mother has accused my brother-in-law of inappropriate advances towards her. Clearly, she is delusional. My brother-in-law has been a huge part of helping us with her caregiving, always helping her with home repairs or dropping everything to run to the store when she out of cigarettes or to show her how to work her heater or TV remote control again. Even driving her along with my sister to doctor appointments. He is very patient with her and treats her like if she was his own mother. My mother has always liked men, she was very attractive and had no shortage of suitors. Growing up she would go through men like a kid in a candy store. Now every time I call her, she will say to me "did I tell you about what happened", and I must hear the story again and again. Clearly this didn't happen, but in her mind it did. My problem is, she is so upset with him, and he is now reluctant to be alone with her, who wouldn't be. This means that she now only calls me or my sister who does not drive for her needs. This is a huge loss for us, his help was very valuable to all of us. How do I explain to my mother that this delusion is just that, a delusion? She knows her mind is not working properly; she constantly tells us she is confused and that she needs help or to put her in a home. She lives alone and is currently on a waiting list through Medi-cal to be placed in an Assisted Living Facility. Has anyone ever experienced this situation and is there a way to help her realize that this accusation of inappropriate behavior did not happen so we all can continue to help her until she is placed in a care facility? Has anyone experienced any kind of delusional behavior of this type and how did you handle it?