What can a minor do to sue parent for child abuse?

I have a friend who I will leave anonymous. We are both minors - he is 15, I am 14. We go to the same school and talk sometimes. He confided in me that he has been physically abused by his mother since he was 7, has had lifelong damage, has broken many bones, and it has been life threatening. Most recently, he came home from school and was immediately grabbed by the throat. The parents are contemplating divorce; the dad sometimes stands up, but when police came as alerted by a youth services counselor, the mother lied her way out of it and the dad said nothing. Based on research, I believe that minors can't sue without a parent or lawyer (though I could be wrong). It's uncertain whether the dad would testify for him in court. I want to know what my friend can do despite being a minor, and how the situation can be legally resolved. If it makes any difference, there are apparently two 12 year old younger siblings and an older brother (17) who have all been abused occasionally, though the younger brother (12) and my friend have it the worst. They have all been verbally abused and harassed, especially my friend. For now, I've encouraged him to record every incident from now on in detail, including time, date, pictures of injury, description, etc. on his locked computer, backed up onto the Cloud so the mom can't find/destroy evidence. In my experience, without evidence you can't get very far in making your argument, and with solid, legitimate evidence it is easier to make your point. Thanks for helping me help him.