Friend's mom is abusing her and there's nothing I can do

My best friend's mother. What can I say? The bitch is worse than what the lovechild of Agatha Trunchbull and Dolores Umbridge. My friend is trying to abandon laziness (and she's doing alot of progress) but it's not an excuse for what her mom does. My mom's friend is constantly attacking my friend; she hits her (I've seen the bruises and cuts), she yells at her for the smallest things (hell, the hostile bitch constantly tells my friend she's worse than the people who molested her),she tells her that she'll never amount to anything, the bitch tells her relatives that my friend is a whore-ish low-life, she's threatened to kick her out, she never lets her leave the house, she's threatened to take her to a ghetto/ dangerous school, and the bitch has the nerve to post shit on facebook like "Thank the Lord for your loved ones and never take them for granted!" or "My daughter ____ is such a hardworking blessing! I'm proud to be her mother and I'd do anything for her!" or some cheesy anti-domestic abuse poster. Hell, the bitch is a social worker who for some reason has not been fired. A FUCKING SOCIAL WORKER. My friend is constantly in fear of her, never knowing what the bitch will do next, and she's becoming much more fragile and lifeless as of lately (her mom is becoming worse and worse). And I KNOW my friend isn't making this up; apart from the bruises/ cuts, I have been in the car while her mom yelled at her. The argument consisted of cussing, comparing my friend to the people who molested her, and calling her stupid and worthless all because she didn't get 3 sodas that were on sale at the gas station. I felt chills and fear through out that ride, and the bitch acted like her actions were justified and she acted like nothing the rest of the night and my friend has to put up with that everyday.
Worst part is, I can't do anything about it. If I call child protective services on her, my friend will be taken away who-knows-where; she's told me she doesn't want to be sent away. We've exhausted every option but there's always a dead end. As of now, she's just hoping to be able to leave the house a few months from now when she turns 18. That means she has to deal with colleges, finding a job, an apartment, and a form of transportation within the next few months. And there's nothing I can do; I feel almost as trapped as she is.
I don't know what to do, and I'm typing this now while she's talking to me about some shit her mom just did.
I don't know how to help her and I feel horrible.