Yes, the so called experts have already gotten to work here I see... However it's true that most emo people(much talk, little action), girls or not, "fail" to commit suicide. They are not all that serious about dying and prefer to do it in ways where there is a lot of chance for getting away alive.
The ones who actually die are those who there seem to be no problem with, actually they might seem like the happiest people in the world to the untrained eye, then one day, bam, they're gone and nobody has a clue. Until they start to remember the little signs, or maybe discover a stash of very disturbing, unaddressed letters in a drawer or something, whatever. The point is that people who really want to die mostly do not talk about it, and when the time has come they make sure it happens correctly and rarely fail.
Sure you should still take this girls talk seriously, but not in the way that she might go and kill herself, more like that she has serious issues and needs help. If you want to help her, don't actually play along with her bullshit, you told her not to be stupid, that was good, see how she corrected herself? So don't start induldging in her talk, you got to counter it and try to drag her out of that hole she's dug for herself. Or she will get more and more comfortable in it, and even if she'll talk about getting out of it because that's what others want her to, it may take a long time until she get around to seriously doing it.
It's good if she cares about you, so first gain her trust and her heart, then you will have her hooked and it might work to forcefully pull her away from her destructive emotions.