There is no public proof; at this point it is a he-said she-said over what happened behind the scenes. I acknowledge the benefit of the doubt in this situation goes to Tanden. Though I do not have good experiences with her truthfulness and tend to disbelieve her on this point, I acknowledge others don't share the background and will likely (fairly) conclude otherwise.

I am hopeful that future Wikileaks releases shed some light on this affair.