Some things you can do for a friend who is being abused:
Most importantly, be there for them emotionally. Tell them/remind them what they are going through is abuse, and they do not deserve it.
If you can provide a temporary space for them away from abuse, do. Have them over to your house, if your home is non-abusive. Even a few hours in a non-hostile space can do wonders for someone’s emotional state.
Ask yourself, or your parents, if you can provide your friend a space in the longer term. I have known many abused children whose first step to freedom was going to live with their friend’s family.
Step in and call 911 or CPS for them on your own judgement. Not their judgement. Abusers are experts at making the abused feel like getting help from the authorities is worse than abuse. This is not true. They will thank you for it when it is all over.
If they are trying to escape abuse, help them by giving them a safe place to keep their property temporarily, especially important documents like Social Security cards and birth certificates, and expensive things like laptops,
Help them research ways to get out. Shelters, jobs in the area, cost of living. Abusers make escape seem impossible. Show them facts that its not.