My best friends dad touched her?

Okay, so my bestfriend just told me she is scared to sleep in her own house, so I asked why? And she said cause she keeps getting touched, long story, short, she told me her dad had been touching her inappropriately, and she made me promise not tell and that she wants to tell my sister, who is her step mom, but she's scared, and she wants me there with her, but my mom won't take me there, so I don't know how to handle this situation. Help shes 15 and I'm 14 years old.