My bestfriend is in an abusive relationship, *Please answer!*?

I don't know what to do. Yesterday she came to my house crying, and showed me bruises on her back, and arms. She said I was the first person she told, and she isn't going to tell anyone else. She said her & her boyfriend got into an argument, and he hit her, and things just got worse from there. I feel like a horrible friend because I don't know what do to! If I call the police, I'll lose my bestfriend, but I'm terrified that if I don't call, I will still lose her!! :( This is serious, I'm not joking, I honestly feel helpless.