TheQuietOne, I have read through this thread many many times and I have wondered what to say. As someone who has been suicidal more times than I can count that when I made the attempt I TOLD NO ONE. If you met her in a forum like this and have no contacts in which to call to get her help then you need to let her go. Defriend/block her in everyway. That sounds really cold but this is distressing you and what she is doing to you isnt fair seeing as there is no way for you to help her. Second if her main therapist knows and has chosen NOT to put her in the hospital then she has a professional who can do something if and when the time comes. If she is attention seeking/exaggerating this way she needs to learn that this is a good way to lose friends. That is an extremely stressful situation you are in. She has been saying this for a while and has done nothing.
I have question given all the FACTS you have given us...what would you do if she somehow covinced you she was serious? You would still be in this very situation...stuck with no way to do anything. Let her go...this is extremely unhealthy and is stressing you out. TAKE CARE OF YOU.