Sad to say, but at 22, she is an adult so there really isn't anything you can do to stop her.
It sounds like she isn't very informed about sociopaths. If she were truly informed, she would know that there is no way she will be able to fix him.
So, while you can't stop her, perhaps you can pass along info that explains that he will always be a sociopath.
I'm hoping that one of our posters who has excellent knowledge on this topic will see this post. If she sees it, she will pass along lots of helpful info to you.
When your daughter finally realizes that she can't change this man, be there for her as she will need your support.