. . .she needs someone to talk to bad. . .
You know, it's a good bet that this is probably why it's whipped up to such a fever pitch in her head. If she has nobody to talk about it to but you (the putative "enemy") then she's probably in some kind of feedback loop that's driven her into a panic. It's interesting that she wants to divorce but won't talk to her family because they will tell her to get a divorce.
You know about Faces East, right? (I assume it's still a functioning board.) Have you suggested that she go talk to people there? Even just reading the stories there would be good, if she doesn't want to post. It might help her to realize that others have gone through this and are dealing with it OK. I think, the little I've read on their board, the philosophy over there is to keep the marriage together if at all possible.
It might help her calm down and see that you not believing anymore is not the end of the world - not anybody's world - not yours, not hers, not your family's, NOBODY's. It'll be a changed world for all of you, but people can do change. SHE can do change. Of course, divorce isn't the end of the world either. But it's a radically changed world too, as people have mentioned. One way or the other change is coming. But she (and you) can choose what that looks like. Maybe getting a sense of control (which is basically denied to mormon women--I think that's why they freak out about this kind of thing so much) can help her get her feet on the ground.