Hi,
I agree with all the other women, tell her to talk to her school councelor, they will hopefully know what steps to take. If not, you go somewhere else. One thing, I would ask the girl first if she has any family here that she trusts or could go to. I ask because what if something does happen and she is removed from the home? I know kids are supposed to go through the system but I have known of cases where they let other family members take the kid(s). I would make sure she has her dad or other family here that she would go to. By no means do I think she should stay in this situation, her mother is selfish and neglectful but I just want you two to be prepared.
You are a kind lady to help this girl. By her mom not kicking that disgusting man out, she is basically telling her daughter he is more important. I have many issues with my stepdad none are of a sexual nature but bad in other ways, my mom has always chose him. I am 30 years old, she is still choosing him no matter what he does and I am still affected by it. Parents just don't realize their how important their roles are. They are responsible for what kind of person their child turns out to be and that should be taken very seriously.
I am very narrow minded when it comes to parenting. I feel very strong about what a parents role is. 'Bad parenting' is what is mostly wrong with our society. Children are raised without love, caring and guidance then released into our community and have no idea how to function properly because of their lack of stability while growing up. My heart just breaks every time I hear of a child being neglected. May God Bless you for trying to help this child. It's small efforts like this that change our world.