Both Mom and Dad have mid-stage dementia. Dad tells Mom he goes out several times a week to dine with friends. (It never happens.) Mom screams at him for leaving her home alone. I try to tell each of them separately that the other is hallucinating, but it doesn't work. What do I do? Any suggestions greatly appreciated.
Is Dad saying these things to get a rise out of Mom, or is he delusional and thinks he did them? If the latter, there is no reasoning him out of this false belief. You MIGHT try to convince him that hearing about his dinners makes Mom jealous, sad, whatever, and that it would be better not to tell her about them. But even if he agrees he may not remember that.
If Mom were cognitively healthy you could probably convince her that Dad is delusional, he is making things up, but he really believes them. But since she is cognitively impaired too, that probably is not going to work.
Maybe your best bet is to head them off at the pass. As soon as the topic of dinners out comes up, change the subject, distract them, redirect them ... just get them off that topic.
Can your parents actually still go out to dinner? Maybe something along these lines would help. "Speaking of going out, wouldn't it be great if you two could go out to dinner together? I hear they've remodeled that place out on highway 5. Shall we all plan to go on Thursday?"