I don't normally advocate lying, but is it possible to tell your mother in such a way to make it look accidental enough you can tell him that it "slipped out"? I know that isn't entirely honest, but it is such an awful position to put you in. My father is rather oblivious and lacking empathy as well, but I just have it out with him when I need to. If it were me & him I'd just tell I am having no part of it because I believe it won't be pleasant and I am not participating in something that I am so uncomfortable about. Does this friend of your mothers have any idea how your mom feels?

Also, is he seeing this friend of your mother's romantically? It seems odd they would opt to travel together to me, but perhaps it's not and I am reading too much into that.

@threetimes I don't see why I couldn't. I'm not sure if that's the best way to approach it, though. As for the friend... as far as I know, he's had no contact with my mother, I've not seen him since I was about 1 year old, and all of his information about our family gets filtered through my father, so I suspect he has no idea my mother might not be friendly with my father.

@threetimes That thought has occurred to me, as the road trip does seem strange. The friend is male, and I have no reason to believe my father is bisexual/pansexual (although I can't rule it out). I am more inclined to believe he (the friend) is either genuinely excited to see my mother than any sort of romantic relationship.

In fairness my father has actual brain damage that has contributed to his poor social behavior. As such, the only thing that works is direct confrontation when he is not being realistic on his thinking of given situations. There is no real way to finesse my father. I've seen plenty of people try. I have tried. So you have to always cater advice I think too based on how the person you are dealing with best grasps your boundary. And with my father it's sometimes easier to get forgiveness than cooperation, so I might do it, then tell him after. Again, my dad though, and I know him.

Did your father ask for secrecy before giving you the information or afterwards. Do you have any pre-existing social contracts that would suggest that he should be able to expect such discretion? If you and your father have a history of being able to confide anything in each other, you may not be in a position to spill the beans.

@CortAmmon He told me first, then said "don't tell your mother" because the friend wanted it to be a surprise. And no, I have not been able to confide in my father for decades.

How well do you know your mother's friend? Have you talked about the matter with him; if not, would you be able to?

@sumelic if I've ever met this friend, it was when I was about 1 year old.

@Beofett If your father wants this other person to be a surprise, could you ask him whether you can ask your mom just about meeting up with your father? If you think your father is enough to make your mom react negatively to a meetup, and you can get something from her that says as much, he may agree to let the friend meet up with your mom as a surprise without your dad?

Why is rejecting your dad's request not listed as one of the possibilities?

Is your father aware of his inability to empathise? Is there reason you can't have an honest conversation like: "Hey dad, remember X, Y and Z occasions where somebody acted very differently from how you expected? Well I think this might be another situation like that..."

@BenAaronson No, he has no awareness of this. He truly believes that every personal disagreement he's ever had is everyone's fault but his. It is why I believe this friend is most likely his only friend.

How are your mother and father going to end up at a dinner at the same time? Who is setting that up?

@Catija I suspect that my father would be confused as to why that would be suggested, but it may be a possibility.

@stannius Presumably me? My father seems to be assuming that there is no worry that there might be difficulty in getting time on people's schedules, which isn't surprising given his empathy issue.

Is your dad self-aware of his own lack of empathy?

@stannius No, not at all.

Well, if you need a reason to justify to yourself (your father won't listen to justifications) to refuse his request or modify it to something better: consider it as a favor to your father, doing something on his behalf he isn't capable of on his own.

With respect to the logistics, you could try to set her up with an after-dinner plan that would allow your mother to simply leave dinner at any point and go straight to the after-dinner plan, to prevent the entire evening from being ruined.

I notice many of the answers helpfully suggest getting yourself out of the middle. At least one circle of therapy theory would call this "detriangulation", and has come up with a set of strategies and techniques for doing such a thing effectively and to everyone's (eventual) benefit.

"it hasn't occurred to him in the slightest that she would be anything other than thrilled to see him" I actually laughed at this. Must be hard. Stay out of it is my advice for all familial conflict. Just smile and let it play out.

@Beofett Well I was wondering if he could be convinced that a) Whoever's fault these conflicts are, he isn't very good at predicting them, b) It would be a good idea to a conflict if he can, even if it wouldn't be his fault. But that probably would be a tough sell.

Can you contact your dad's friend, and appraise him of the situation? He may be able to negotiate the possibility of discretely meeting your mum while your dad & family share a meal.