I agree with Misty. Start documenting all the things that they are writing in a public space. Don't write to her, it's up to her to figure him out and they may even be a perfect match - you'll simply be seen as a jealous ex who is trying to keep your daughter away to hurt him. Your daughter is obviously old enough to make her decision, the fact that he isn't turning up to mediation is silly on his part, as is openly defaming you. Don't buy into it, continue to be the good role model for your daughter and let it play out.