I don't know how to handle my partners poor parenting.

My BD is 6 years old, her step father has been in her life for 4 1/2 years. They've generally had a good relationship.However in the last 2 years he's started to become like ultra authoritarian. He speaks to her in a harsh tone (my parents have noticed and spoke to me about this and everything else I am about to say), had told her he doesn't like her, pulls her up on some really inconsequential crap and has begun to make dinner times an absolute upheaval (I now just tend to eat with her before he gets back from work)I'm struggling with this as she's now back chatting in the same manner he talks to her, which naturally he doesn't like, so tells her off. He doesn't tend to listen to her (she can say daddy 4+ times and he apparently won't hear her even when she's stood next to him) he doesn't particularly engage in any real conversation with her except to tell her off.I've sat down with him after bed time spoke about the fact that he doesn't listen to her so it makes sense that she sometimes doesn't listen to him, that the tone in which she speaks back to him is a reflection of how he talks to her, that you can't tell children you don't like them (which caused him to start ranting etc).I've suggested us reading up on ways to discipline together (which was met with I don't need the internet telling me how to discipline a child- it's working out really well for him) I've told him he needs to remember she's a child they push boundaries and it's our job to guide them.I hold my tongue and try to talk to him about it away from her but I'll admit I do call him out on it in front of her sometimes. It's got to the point where I dread all of us being home together because I know that it'll end in misery and probably an argument of me saying it's unacceptable to treat a child the way he is and him telling me I am wrong.I know he sees me as undermining him but I have spoken to him privately so many times, explained the lack of listening etc on her part so many times and he doesn't work to do any of the agreed suggestions.It's frustrating as generally speaking she listens to me, isn't rude in general to me and when I correct her behaviour she just gets on with it. As far as a 6 year old can you know?Our daughter is a wonderful child, she is of a sunny disposition, interesting, polite and just really nice to be around for a six year old. via /r/Parenting https://ift.tt/2Otblqh