My father has dementia. He's accused my mother-in-law of having an affair with her daughter's husband which is totally untrue. Advice?

This is all part of the desease, its paranoia. Nothing you can do or say will change his mind. Its funny how they can't remember what you have said to them a minute later but they get this one thing in their mind and can't let go of it. They can no longer be reasoned with because they have lost that ability.

I agree, if SM visiting causes him agitation she may want to not visit as often. May be good for her too.

Sadly this is a common delusion. My widowed aunt started thinking this about her deceased husband, saying “he was out running around on her and was never home.”

He was dead.

Considering your father's mental state I strongly doubt he can get a divorce. There is also nothing you or your mom can do to disabuse him of this notion he has - this is the loop he is in - don't reason with him, don't argue with him, just give him vague answers and distract him to something else. It's part of his broken brain. At some point he will most likely move onto another subject. At this point it may be best if mom visits a little less and hopefully dad will move on eventually. It's hard on you and especially your mom - but remember his brain no longer works the way it should.

My dad had his delusions but once he told me mom was out spending all their money and had boyfriends - I believe I told him I had their money and mom wasn't out there spending it - and for the boyfriends - "aw dad, you know mom doesn't have any boyfriends. " I don't think he believed me - but he didn't push the issue either - dad was still in the mid stages and could sometimes accept an answer. Dad died about 2 years ago and I recently told mom of this interaction - her response was I know he told me too. When I asked what she said to him she said "I told him, you know you're the only boyfriend I have." He died before he got any real entrenched ideas that we had to live with till he moved on to something else.

Sorry no real advise on this one - just a sympathetic ear. Blessing on all your family and a wish that your dad soon moves onto something else.

This is part of dementia. Given time he will move on to some other thing and be obsessive about that. However for now best to ignore it and change the subject. Caregivers in NH have seen it all and know he is confused.

Do you mean your stepmother? I dont see how a MiL could put you Dad in a NH..

Regardless, you pretty much just have to nod and smile. You're not going to be able to change a dementia patient's mind. A non existant is a fairly common delusion.