Most Helpful Girl
Just let it go.
He's badmouthing you because the relationship ended just days ago. He's in the recovery process and like it or not, badmouthing is a common recovery method.
As for why they're even talking when they used to not like each other? It should be obvious...they both have something in common now: you as an ex. They can bitch and moan about you together and it will make them both probably feel better. And if the former ex is over it, at least he can probably understand some of the things the latter ex is talking about when he rants about you.
When he gets over it he'll stop.