First, get them out of the situation. If they don’t have somewhere to go that the abuser doesn’t know about, see if you have a local women’s shelter. Leaving the area entirely, at least temporarily, may be a good option.
Second, help them take action about the abuse. Contact the police or find a lawyer to help with the process. There are lawyers who will work cases like this without charging. There may also be local government agencies that are specifically tasked with investigating abuse.
Third, get them to seek help in recovering. There are likely to be local organizations that provide counseling and other resources. This is not going to be a short or easy process, but working at it will make a world of difference over the rest of their life.
I’ve been through this with more than one friend. Every bit of it is tough. Having your support will make a big difference. Abusers try to make their victims feel powerless and alone. Having a friend who will help works against that, as well as providing comfort.