Question Regarding Domestic Abuse..

I have a friend, & her husband is physically abusive. She tries to get out, but she has nowhere to turn. The home is in both of their names & she is a teacher & she's just stuck. NOW, here is my question & concern. Her husband abuses her weekly... Today, he grabbed her hair & drug her across their yard. They have a metal chain link fence.. He drug her to the fence & slammed her head into it where the latch is over and over again. Her head is busted too pieces. She was swinging her arms around scratching and pulling and hitting at his face to try and break free. 
She tries to call the police, but every single time, they tell her "if we take him to jail, we have to take you to jail with him because you hit him also," "it's his word against yours" blah blah blah... She does it in self defense. I truly believe he would have killed her by now if she wouldn't defend herself. She can be dripping blood and he will have no scratches on him or anything and the police will say if she tries pressing charges and they take him to jail, he also has that same option and she will go to jail too. 
She's terrified. She's never even had a speeding ticket, and she sure doesn't want to be arrested. She is a 6th grade teacher. She is a mother of a 20 year old. She is a good person. 
Is this legal? Are the police even making any sense to you!? This is happening in Missouri. He has a lot of ties within in the community and we don't know where to turn.