Custody statment with out knowing the mother

So my ex's ex feonce before me got in contact with me saying she has just heard I'm taking him to court for an extended avo. Its a very long and messy situation. She has come clean with me the abuse he put her through and still is and has offered to write something for me in court. She then went on to tell me that he is trying to take their daughter off her by manipulating her. He has a new girl and the daughter is made to call her mum. She now says she loves her more than her real mum. He has told the daughter to tell her teacher her mums new boy friend hit her which he didn't. She is very worried about her. He wasn't very nice to her when I was with him. When I left he filmed her saying to her I don't love her thats why I left (she was 3 at the time, now 5). I told her some of the things he use to do like yell abuse at me in front of her she would always watch me cry. Or how I did absolutely everything and he did noting but just yell at her. She asked me for the videos to show her lawyer. And wants me to write something up for her when she goes for soul custody.
I said I would. I have always worried about that little girl when I left so I'm really happy to help out. But even though I was with this man for 2 years I never had anything to do with the ex feonce he wouldn't allow it. I had lots to do with the daughter how ever. How do I write something up with out knowing her. We have been talking for a while now over Facebook and want our daughters to meet as they are half sisters. (I left when I was pregnant). How do I start the statement. What do I say? Thanks in advance.