Ok, backstory is Ex and I separated five years ago. I left the relationship for many reasons. Ex has had issues with depression and alcoholism during and after the relationship. We have a DD together.
Ex and I have always had informal shared custody. We both went through difficulties when the relationship ended but we strived to have a solid friendship between us for the sake of DD.
It hasn't always been easy, and I have had to help Ex out. For example, he was having issues with alcohol, and because he was binge drinking I helped get him some professional help and into rehab. He didn't have contact with DD for about six months during this period of his life due to dealing with therapy etc. I kept in contact and helped all I could so that when Ex was ready he could slowly get back a good relationship with dd. He also occasionally relapses during which time he will just go missing. I normally can tell as he sends odd messages just before going AWOL.
There have also been other issues which we have worked through, on the basis that we were friends and to maintain a good relationship for DD. Ex works shift work and as I have DD the majority of the time I was happy to fit in contact whenever ex was around/what would fit in with his ever changing working hours.
I was overjoyed when Ex met someone not long ago. He chatted a lot about her initially and he was really happy. Unfortunately, just before christmas it started to go wrong. Ex became really distant and cancelling contact sessions with DD last minute - which really upset DD. This has gone on since the start of December all for flaky reasons, and is still going on at the moment (he cancelled having DD this weekend due to needing a date night with girlfriend - despite having other nights off during the week).
Anyway, things between us have become strained and in December he mentioned that his girlfriend wasn't happy about our friendship - I said I appreciate why she would find it difficult, so would back off - ie. Ex and I used to meet up for coffee when we were both free and DD was at school. I thought that would be ok, especially as they moved in together just after Christmas.
That brings us up to recently, when DD got very upset this week. Crying her eyes out and unable to be settled. I asked what was wrong and she said that she was sad her father and I were no longer friends - I stated that we were still friends and that there was nothing to worry about, things may have changed with her dad but that was a good change and he was happy. That's when DD stated that her father has regularly been saying quite vile things about me. Ex basically is calling me a bitch, saying I make contact difficult and that Im not over him. He also says I'm a snob who has issues. DD says that these things are said to DD, in front of his girlfriend. Sometimes they are said directly to the girlfriend, whilst DD is in the room.
DD has begged me not to tell her father that I know what he has been saying as he is very strict and would shout/hit her for telling me. I have always had to ask ex not to discipline too harshly, and normally that would be respected. However I have a feeling that anything I said at the moment would fall upon deaf ears.
I'm struggling with this as I have no idea how to proceed. Ex is proceeding to be difficult at the moment - I mentioned it was the holidays two weeks ago - did he have any plans that I should work around as I want to organise things for DD. He said he wasn't doing anything and it was fine, he'd work around me. Today I receive a text telling me that he wants DD all next week until thurs 
. I have made plans and have told him so, but he is now insisting he can have DD when he likes.
I must admit I'm livid with him at the moment, as he's upsetting DD and, it feels like he is trying to turn her against me. Also I do feel hurt that our friendship was trashed so easily by him when I have gone out of my way to help him in the past.
Sorry for the long post, but what would be the best way to proceed with this? I can't tell him what I think, as that would break DD's confidence in me, but equally I'm struggling to be civil towards him at the moment.