My friend's stepfather beats her and pulled a knife on her...?

The question pretty much sums it all up... her mom is always making her cry whether I'm there, or not. And she told me that when her father did all this to her her mom just egged him on... So I don't know what to do. I really don't want to just pretend that this isn't something important, because it's serious. I can't lose my best friend. Should I tell my own parents? Should I talk to my school counselor even though she doesn't go to my school? Is there some sort of help line I could call? I live in Scottsdale Arizona if that helps. Should I just tell her to do something about it? I don't want to make anything worse, I just want to help. Thank you, and God bless<3