When Sansa and Arya reunited, Sansa says that she wishes that she had killed Joffrey, and then points out the memorial to their father. In the letter she calls her father a traitor and refers to "my beloved Joffrey". I think Arya will be smart enough to smell a rat, particularly since Littlefinger has been handling the letter.

My guess: Arya will be pretend to be taken in, have a meeting with Littlefinger, and out comes her knife.