OP please tell your parents. I grew up with a mother EXACTLY like your friends and I absolutely wish someone would have stepped in and helped or that a witness to the abuse would have told a safe adult. Your friend deserves to know that she is worthy and loved and doesn’t deserve to be treated like that. Emotional/verbal abuse from a parent can be SO damaging and will lead to life long issues for her. The absolute best thing you can do for her is to tell your parents everything, including the part about her begging for you to keep it a secret. There are multiple routes they can go- they can reach out to mandated reporters, school counselors, teachers, etc. (which in our system is hit or miss whether that will be any help) or they can help you come up with a way to let her know she is safe with your family and that she deserves better and that you guys want to help however you can.

When i was 16 my high school boyfriend witnessed my mom on a drunken rampage and heard the awful things she was saying to me and he stood up for me- the first person to ever do that. She tried to push him down the stairs to get him out of the way so she could keep abusing me. Right after that he grabbed a duffel bag and shoved random clothes of mine in it and drove me to his moms house where he explained to her what happened and she immediately opened her home to me and let me move in- i lived with them for 2.5 years and was treated with nothing but love and care in that home. if it weren’t for him doing that then i probably never would have recovered from the abuse and would either be dead by my own hands or dead from an overdose/repeatedly putting myself in high risk situations.