That sounds like a very immature attitude. I have to agree with Tim. Obviously she doesn't want people, you included, to think that she has breast implants. Maybe she's suddenly ashamed of them, maybe they didn't turn out like she wanted, maybe it's something to do with her husband ... who knows? It could be anything. The point is, your friend doesn't want you to know, to the point of being immature about it herself. If she is your friend, then be mature and let her have her privacy. Drop it for both of your sakes. Needing to be "proved right", to expose a friend's relatively unimportant lie/self-deception/whatever ... what purpose does it serve? To prove your own moral superiority? To lord it over her?

If you're afraid that the lie is an important one that affects your friendship, then you have to tell her that and be prepared to not be her friend anymore. If it's really not that big a deal in the scheme of things, then man up and get over it. That's my advice.