Mother has delusions about infidelity.
My mother is 96 and thinks my 90 yr old father is having an affair. She had made up all these stories about how this woman calls my father and spend the night with him. Mom lives with me because she had had several falls and I've been her caregiver. My parents home is a two story with the bathroom upstairs and the it would be expensive for them to have the house remodeled. Plus, my dad wouldn't be able to care for her. Mom talks of nothing else but this affair. She cries all day, won't eat more than one meal, and gets so agitated at night that she can't sleep. She asks for dad to come over and says she misses him, then when he's here soon she does is make ugly accusations and verbally abused him. Yesterday he came by because he misses her and again but she kept up the verbal abuse and she all evening she cried and begged me to call dad and tell him to come back. When the paid caregiver is here, mom will call dad 7 to 10 times. Dad won't answer the phone because it's upsetting for him. We worry about his mental health as well. I can't reason with her and she thinks I'm against her. She's on Quetiapine and Lorazepam. Last night she refused to take her medication until my son finally convinced her. Then she started coughing and choking. Not sure if those pills went down. Respite for me? That's a joke... I still come back and don't know what to do.
Convincing her that her suspicions are unfounded hasn't worked. I doubt they will. I might confer with a mental health professional to get specific advice about what to say to her. It's a huge issue with her. I can't help but wonder if she would settle down, if told that he made a mistake, stopped the affair and begs for her forgiveness. At least, she might be able to let it go. Because, it's so painful, I'd get advice on that approach from a trained mental health professional though.
ALSO, keep in mind that the reason your mom may keep going on with this affair belief over and over and will not let it lie is that she FORGETS that she has already done it the day before and the day before that. So, she may keep doing it, thinking it's the first time she has confronted him about the affair. Nothing can change that, because you can't make her recall a resolution if she has no short term memory to recall it with.
Unfortunately this is a fairly common (and heartbreaking) delusion. Please keep us posted on how this works out. We learn from each other.