Father accusing mother of infidelity?

Ah. Your profile says dementia so that is what I assumed. I suppose this kind of paranoia could be something else, but whatever it is, it is not your dad being his normal self! It will be good to have him evaluated.

Thank you for replying to my ambiguous question - (I didn't realize it was going to post before I could explain further.) Nonetheless I appreciate your response. I suspect my dad is beginning to show signs of dementia. He has not seen a doctor about this yet - however, I'm in the midst of speaking with a geriatric specialist about getting him evaluated.
In the meantime, any advice is helpful, especially for my mother who is obviously feeling very hurt by these accusations. I also worry about her because he becomes angry with her easily.

Oh how very, very sad! This is not at all unusual behavior with dementia. It must be terribly stressful for all of you.

In general, it is considered best to go along with delusions and not to argue about them, but obviously going along with this one is not going to help anything! Still, try not to be argumentative but rather reassuring and comforting.

"Oh, sweetie, how awful you must feel to think I (or mother) would do such a thing after all we have been through together. That must make you feel hurt and betrayed and angry. I don't know how that idea came to you, but I assure you I love you very much. I will never abandon you. I don't have any boyfriends. You are the one I've always loved." Etc.

It may be of some comfort to Mother to know she is not alone, and to know that it is definitely the dementia that is causing these accusations.