First, I am so sorry because I know the grief that this must cause you. You must feel very powerless and afraid for your loved one. However, since she is presumably a competent adult (albeit one who is being controlled and possibly terrorized by an abuser), there is nothing that you can do legally. I have known many people in this horrible situation and essentially it is a powerless and very frustrating position.
A victim is most likely very afraid to do anything to defy her abuser since she knows he can probably try to have her wrongly committed, leave her financially unable to survive, have her falsely arrested and/or take custody of the children if she defies him or leaves him. This is why many abused women choose to stay until the children are out of the house. Things sometimes get even worse and harder for them if they try to leave.
If you try to take control, you will most likely alienate her and cause her abuser to isolate her even further from you. I'm sorry to say, but at this point, unless she asks for your help, you must just do your best to keep the lines of communication open with her and let her know that you are there for her when and if she is ready to make a move away from him.
Since he may be monitoring her very closely, I would not talk with her on the phone about her situation or send any e-mails or texts to devices he has access to. I would only trust in-person communication with her, away from the home where she may be recorded. The more information he has about your attempts to interfere, the harder he is likely to isolate her from you.
I suggest that you contact the domestic violence agency nearest you and see if you can arrange to meet with an experienced domestic violence advocate so that you can receive support and have a sounding board. Often friends, counselors or faith leaders have very little understanding of the nuances and safety issues involved with domestic violence, so it is important to talk with experts about how to deal with this. You may also be able to reach out to your state's Coalition Against Domestic Violence for information or support.
The hardest thing for a friend or loved to do is to work on surrendering the tendency to try to manipulate the situation. As heartbreaking as it is, this is what you must do until she is willing and ready to reach out. If you try to coerce her (which is what he does), you will likely push her away because she fears your interference will endanger her or make things harder for her. This is what happens with traumatic bonding or Stockholm Syndrome. (The book Trauma and Recovery by Judith Herman MD can help you understand this.)
As a loved one or concerned friend, you may be under much stress because of your concern and so I highly recommend that you do whatever you can to address your own self-care needs. I urge you to develop some practices which contribute to your own emotional well-being and peace of mind. You may be at high risk for what is known as vicarious trauma or compassion fatigue. I hope you will find some ways to take to care of yourself and find some inner peace as you deal with the heartbreak of this very difficult situation.