What do I do about my friend?(serious help!)?

Okay, so there's this girl named Jen (let's just call her that) and she get abused at home by her mother. She told me this a week ago and it makes me cringe how a sweet person like her can have this going on at home. She told me not to tell anyone, but I feel the need to have to tell someone.

I saw her yesterday, she had a bruised back and her hair was cut. She had bloodshot eyes like if she had been crying, and I tried asking her about it, and all she said was "My mom apparently thinks I'm a dog." and then she left. I asked her to report it, but she denies it, saying that she's(her mom) the only person in her family she has left.

I just don't know what to do, and I keep reminding her "Better to be alone than in bad company," but she wont listen.

What do I do?

Thanks for your answers, and have a nice day.