My parents have been divorced for roughly 40 years. The separation was unpleasant, as divorces frequently are, but so were the following decades. 

They have not spoken to each other outside of a handful of words during my wedding.

There were courtroom fights, in-person fights, and everything in between.

My father has announced (not asked) that he will be driving halfway across the country to visit with me and my family (by which I mean my wife and son).

The plot twist:
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He will be traveling with an old friend of my mother's. He has stated that the two of them would like to have dinner with my family, my mother, and my mother's husband. **He has asked me not to tell my mother**, as the mutual friend (someone that my mother has not seen in more than 40 years) wants to surprise her.

I have no reason to believe my mother would not be interested in seeing this friend.

I have **every** reason to believe that she would want to avoid having to see my father.

It is probably worth noting that my father has no empathy (and yes, this means he almost certainly has a personality disorder). I mean that in the most literal sense: he seems fundamentally incapable of seeing anything from someone else's perspective. As such, he cannot imagine that anyone that he is interested in spending time with would not be equally interested in spending time with him. Despite the decades of bitter fighting between him and my mother, it hasn't occurred to him in the slightest that she would be anything other than thrilled to see him.

If I tell her, and she says "no" (which seems almost guaranteed), my father will blame me (for telling her, and possibly for sabotaging what he imagines will be a pleasant and social reunion). If I don't tell her, I run the risk of her blaming me for being part of this (and possibly wind up sitting through an incredibly awkward dinner).

What do I say to my mother? What do I say to my father?