Claire.
You're the one that is closest to her, right? If you said something to her, maybe she'd listen. Don't you think that your opinion would matter most?
Unless you really do want her to go through with this. In which case, I hope you're happy with the fact that you're condemning her to a potential lifetime filled with a lot of unpleasant feelings that she definitely doesn't need in addition to the obvious ones that she already has.
Come on. You need to clear your head. Take a step back. Wipe out that personal investment in all of this and look at it from an outsiders perspective. This will kill her. Not in the crazy physical way that you can avoid, but on the inside where it truly counts.