J.S.
Wow... she crossed so many lines that she stepped over the line, wiped it and destroyed it, kept going and can't see it any more. ::shock face::
I would have a really, really hard time trusting her and not allowing this to affect the friendship. I would probably confront her because it's just so strange and let her know that it wasn't okay, and that if she ever has any questions or concerns that she should ask you directly.
After this I wouldn't share much of anything personal with her and I'd probably cut back on spending time with her. If only to make a point that this was wrong and to give her a chance to think about what she did so that she can try to make it up to you.