So refuse to mediate. Tell them they're both adults who can/should sort out their own problems. Even if they're people you care about, it's not your responsibility to fix problems for them.

Um, I won't be asked to, I want to, because I think if I don't then they won't fix things and one or the other will be unhappy with the outcome. And I want them both to be happy. And I think there are things that they each wouldn't think to mention to the other, which might be better known by both.