Dear Daniel:
Sometimes it is easy for me to see what one should do and this is one of these times. Clearly, you should not proceed to a boyfriend/ girlfriend relationship with this woman. Even if she was not your best friend’s girlfriend or ex girlfriend, you still should not enter such a relationship with her because she is very unwell. It is a bad, bad idea to enter an intimate bf/gf relationship with a woman who stated that if you end a relationship with her she will commit suicide (“she said ending our relationship would be last nail in her coffin”).
Do you really want to enter a relationship in which a threat of life and death hangs over your head?
anita