This isn't about whether or not to tell, or how to tell. The issue is how to tell with the expectation that she will honor your wish to keep it confidential. How to do that? Secure her promise BEFORE telling. If she can't promise to keep secret what you have to tell her, DON'T. If she promises but you don't trust her to keep that promise, DON'T. That, however, is a problem itself.
