Just call the police. That's all you have to do. I don't know if there is a special department within law enforcement that handles a case like this. But let the police worry about that.

Let me tell you something... last year, I reported my best friend's niece for physically abusing her 3 month old daughter. It was one of the most difficult things I had to do. I've known my friend's niece her entire life, and she is a troubled individual. My friend told me about the abuse but didn't know what to do about it. She didn't want to report her own niece. Once she told me I was conflicted, just like you. But I knew that her daughter did not deserve that treatment. I called and reported the abuse to CPS (child protective services) and did so anonymously. A social worker was sent out to the home and the mother got help. Today they are doing better.

I know it's not the same situation, but that little girl could not stand up for herself, and neither can your friend's workers. Be that voice.