If I were you, I would stay out of it. Don't mediate. Let them meet each other without you.
If you take part, I would bet at some point one of them will accuse you of taking sides, whether it is true or not. And then you will be deeper into the mess. I would instead tell each of them that you are friends with both of them, and will remain so no matter what happens -- but you are staying out of it. Each will have to accept that you are friends with the other.
I'm embarrassed to admit that I was a feuding part of such a predicament. I dated a girl that had a mutual friend with me. And it ended UGLY (not my fault -- really
). I couldn't even stand the sound of the woman's name being uttered afterward, which made it really tough on our friend. It took me well over a year to "normalize" relations with the friend. And yes
, I still hate the witch, six years later.
DON'T GET INVOLVED.