I was in this problem last year. Tell a superior, a councilor, anyone who can help her and explain that you think she might be sucidal. Then you have to do it. Do it gently, but let her go. It's hard, but do it. And then don't ignore her after but don't see her either. Talk online but try to keep it to a slight min. That way she has the comfort. However if she tries to pull you in again, stay firm. Explain this is for the best. She probably won't. Don't worry. Have someone on guard just in case. Tell her best friend.
Good luck.