Tell her that it's forged, you KNOW it's forged, and you have IRON-CLAD PROOF that it's forged, but that you "promised not to reveal your source." If she presses you, say "Look, I told him I wouldn't reveal his -- the -- name, and I'm not going to."
Let her think it's a male -- paranoia can work in your favor. There is sometimes power in them not knowing what you know, and what you don't know . . . you know?
And just for fun, you can even throw in a "it's amazing how quickly people will talk when they're deposed, under threat of perjury. Sure you want to take that chance? You'll be hearing from us." (and hang up)
