I posted my first reply after reading your initial post, but after reading through the entire thread I have to say that as a whole I am very disappointed in how many (but definitely not most) of the posters would deal with this situation.
Let it be? Come on, that's not an option. This girl didn't run a stop sign when there were clearly no cars around; she knowingly forged signatures, fabricated letters, and appears willing to accept money based on these documents. This is NOT okay. I don't care if it's "a rat race" or if "nothing will happen, anyway," this is about doing our part to make the world a better freakin' place by holding each other accountable.
Go straight to the authorities? It sucks that you got this advice and then apparently used it, because you shouldn't have to be in a position where the authority makes YOU feel guilty and treats YOU like maybe you're the liar, because you shouldn't and you aren't. I truly think that attempting to solve this informally, with the condition set in place and known by her that you would eventually go to the authority figure if need be, would have helped.
Good luck, and I'm sorry she put you in this position.