Most Helpful Girl
It isn't fair to your brother to keep him locked in a relationship (or in relations) with a person just because she isn't healthy. Honestly, that's like someone having to stay because they threaten to commit suicide. And if something were to happen to HIM, she would have to learn to cope regardless.
He can do what I did: help her get settled with a new support team, check on her periodically, and as she gets better, start weening her off. Like an addiction. She sounds very dependent, so it will literally be like weening her off of a drug. Unfortunately relationship addictions are comparable to cocaine addictions :/ Not near as physically traumatizing, but just as hard to quit...
I think your bother is in the right and needs to do what he feels is best. I have been codependent for years, and it is really hard to get out... but I finally had it with my last guy.