How Does She Handle This Mean Woman?-Other People’s Problems

Other People's Problems is about a mean step mom. A woman's friend is mean to her stepdaughter but the woman's husband has no idea.

Mike & Kate-
I need your help. I feel like I'm in the middle of a situation and I just can't sit by and not do anything.
My really good friend got remarried about a year ago to a single dad. He has a 12 year old daughter that he has 50% of the time. She is a really sweet girl who I feel so sorry for.
When my friend's new husband is around she is so nice and kind to her step daughter. When he is not around, my friend is awful to this girl. She is mean and hurtful. You can just feel the hatred she has for this little girl. She is borderline emotionally abusive the way she speaks to her. The tone of voice is never kind. She is always criticizing her and saying complains to me all the time about how she can't stand this child because she is "just like her mother."
The problem is my friend has her husband completely fooled. He thinks she is an amazing step-mom. He'll make comments about how lucky he is to find someone who loves his daughter as much as she loves him. My friend has this guy completely fooled. And I think she has scared his daughter so much that she would never tell her dad how my friend really treats her. This little girl went through a horrible divorce with her parents that I think she is terrified that if she says anything her dad will send her away.
My OPP question is, what should I do? I am worried that this man won't believe me. My friend won't listen to me. Should I mind my own business or get involved?