Sorry to hear you've had such news as this Jon . Never a good thing to have to be on the receiving end with this sort of thing. I've been there myself.
You can never tell what's truly going through someone else's mind at any time. But you can't blame yourself that you didn't go round at the end of the day, you weren't to know she was going to do that
I think if she had really meant to go ahead with her plan, she wouldn't have contacted anyone, she would have just seen it through. The fact that she called you is a sign that despite she wouldn't tell you what was wrong, it was in a way, her way of "crying for help", but sounds like in the state she was in, she couldn't or didn't want to say what she's really going through.
Also with her being found, it sounds like she may have contacted someone else.
The good news is she was found and will now get the help she needs. You though need to be careful how you handle this yourself as well as you both for your own sake, as you can be draw inadvertently into their 'dark place' as it's difficult to know what to do for the best.
Try taking to the professionals if you can to see where your friend has been coming from, may help you decide what you need or want to help out with, if you do?
I'm no expert, but hope this helps you in some way? If you need to chat you know where I am
