I say step back. It isn't the easiest thing to do, but it might be the only real thing you can do. By all means call the local mental health people, but they can't do anything unless she is a "imminant" (sorry about my spelling) danger to herself or others. When my mother was having serious mental issues and my sister was on the phone daily with regional mental health service, it took a 911 call from strangers to get her the help she needed.
