Talk to your parents if you can, talk to a school counselor. You don’t even have to say who you are talking about, just find out what can/will happen if the situation is reported and investigated. The policies may vary from state to state.
When I worked in childcare, we had yearly training on mandatory reporting. Part of the training involved a description of the investigation process, and we were told by the CFS worker who attended the training that they always ask the parents for contact information of nearby relatives or close friends (a support system is how he put it) because they see that as an option to consider before resorting to foster care or a group home. He also said that the child is only removed from the home if they have reason to believe the child is in immediate danger.
From what we were told, the investigation includes interviewing the parents, the child, the child’s teachers, and/or the child’s doctor. If the investigation shows reason for concern, they don’t just whisk the child away but will first try counseling and other options. Also, the parents have rights, such as to deny the CFS workers the right to speak to other children in the family (however, I think I remember him saying that CFS has the right to talk to the child involved without the parents’ knowledge or consent).
If you do report suspected abuse, you can ask to remain anonymous to the family, but I’m not sure if you can not tell the “authorities” your name or relationship to the family.
So, learn about the process and maybe visit the website for your state’s CFS, or CPS, or whatever it’s called there. Talk to your friend about what she would like to have happen. Maybe she doesn’t want her parents in trouble but would benefit from talking to an adult she feels she can trust. It’s a weighty decision, no doubt about it, and your friend is lucky to have someone to care about her.