First off, I'm a bit shocked at the suggestions to just outright leaving. From what you're saying abandonment and loss of trust is a MAJOR source of what's happening. Just walking out is probably going to exacerbate the problem, especially if she's never confided in anyone before. At the very least make SOME effort to get her to professional help,
Since she told you she's never talked to anyone about this before she may be afraid of talking with a total stranger and that's one reason she refuses to seek professional help. Having a familiar face who she's already confided in at her side may make it more comfortable, even if not easy, and might be the difference.
Offer to go with her, sit through it with her. Telling someone to do something is easy and anyone can do that. It would be MUCH more meaningful and convincing a gesture if you actually participate. Not just driving her to the appointment, but actually sticking around, sitting next to her and being there for her. And IMO, if you really want to be with someone that is going through this, you HAVE to be prepared to be involved wherever necessary. Ultimately she has to take care of this herself, but that doesn't mean she needs to go through it alone. She's going to need support, and if you can get her there you can learn yourself exactly what that means, and what you need to do.