Your friend is very lucky to have you. You are thoughtful and caring of her emotions, and this is exactly what she doesn’t have at home. With an abusive stepdad it means she also has to live with a mother who allows is abuse. Your friend has no real support at home because one parent is abusive and the other condones it. Your friendship is an important part of your friend’s ability to see this through. I admire your caring and love for your friend.
The best course of action for now is to support your friend and offer to go with her for some help. There is an organization called Alateen that helps teenagers cope with family members who have problems with alcohol. Her acknowledgement that his abuse often occurs after drinking is what qualifies her for this organization and your connection with your friend is enough of a qualification for you to go as well.
I’d look up on the internet where there are meetings near you and go together. No one there is mandated to report the abuse. But these groups are typically highly helpful in navigating the feelings that come with an abusive alcoholic parent and a neglectful one. In this way you can help your friend understand three things. First- that you can be there with her as an active source of support. Secondly that she is not alone. Sadly, many other teens have experienced what she is experiencing and they have been able to support and help each other through this 12-step program. Finally, when she is ready to break away from her family and move on to a mentally and physically healthier life there will be guidance.
I will also encourage you to call the local woman’s center. You can do it his anonymously to find out what resources they have. Again, sadly, there have been many abused and fearful children of abusive fathers and fearful mothers. They have real experience and counseling and support that can guide you further in helping your friend.
Your love and concern is a tribute to your friendship and your own health and well-being is very important. Please be sure when you talk to the woman’s center to express the difficulty behind having to keep your friends secret out of respect for her. This is a real concern for your well-being, and they will have some suggestions for you to take care of yours.