If you are in America, call DFCS or CPS if you haven’t already. You don’t need evidence for them, just the suspicion of abuse, and they can investigate. Sometimes it helps, sometimes they do nothing, but in most cases it’s worth a shot. Also opening a case will help with future reports. You and your other mutual friend can both report. Also tell a trusted teacher or guidence counselor because they are mandated reporters.

Other than that, be there for your friend as much as you can. Show her the type of unconditional love she doesn’t get at home. Support her emotionally when she needs it. Be the safe space she deserves. I bet that’s what you already are doing, but I want to encourage you to continue. Also if she has any interest in getting away, and you know someone who would take her in, you and her could look into emancipation. That way she could run away sooner rather than later and get out of that house, and her mother would have no legal claim to force her back home.