You have to realize that the threat is a means of control. You also have to realize that if she does go through with it, none of that is your responsiblity. The ones that threaten generally (and I do say generally) do not go through with it.
My ex-husband threatened to kill himself if I left him. I left...he is still alive. This is manipulation at it's best, don't fall prey. Be honest with her and be clear. You will not allow yourself to be held hostage in a situation you are not comfortable with.
What I would like to know is...how old is this girl?