I think you have nothing to loose by trying. It can be used to set clear standards of behavior that are acceptable. Of course they must be reasonable, and you must stick to it.
In my case, I said she had to go to the rehab place and do what ever they told her. I didn't demand an in house 45 day program. I only demanded that she go to the place. Today. And she did. And my life is better for it.
Of course she quacks that she would have done this without my drastic "intervention". Ha Ha. And I can have Abs of steel in only 3 weeks with the $19.99 electric ab stimulator while I dance to Zumba! No amount of previously civil conversation ever stimulated a change in her behavior. Even her kids sitting her down and telling her how much they loved her, but explaining how much they hurt her did nothing. This just shows how out of control and unreasonable they are.
You can not be reasonable with unreasonable people. It is impossible to work honestly with dishonest people. It is impossible to find root causes of problems with the blameless.