MIL in abusive relationship

My mil has been in an abusive relationship for the past 5 years. In the beginning she would text my SIL and tell her she scared and if something happens to her this is her location. We have met this guy but she always told us never share any personal information with him such as where we live, work, etc. She said that he threatens to hurt her kids and grandchildren if she ever left him. It got to the point where he has completely isolated her from everyone, she no longer comes to family gatherings or grandchildrens birthday parties. Everyone is tired of her excuses to leave him and basically gave up on her. She has made plans to escape and at the end never follows through with it. I don't want to give up on her,I just wish I knew how I can help her. If anyone has any advice I would greatly appreciate it