Walter Mabry: This is the key to every computer system on the planet. I want *you* to steal it for me.
Arthur Tressler: Hell is going to seem like a day at the spa by the time we're through with these Horsemen, wouldn't you say?
Walter Mabry: You had me at "Hell."
J. Daniel Atlas: We are going out with a show people will never forget.
Answer: The only ones who knew the chip was fake were the Horsemen, no one else did. Walter did not know the chip was fake. The plan was to convince Walter that the chip was real so that they could lure him out of hiding with Walter attempting to kidnap and later kill them, and expose Walter to the public in doing so. They told the crowd after exposing Walter about the chip, because while it was a fake, they still had plans with it assuming it was real. If by "alert the FBI", you were referring to when Dylan gave the FBI director the evidence on Walter and Tressler, that was not the chip he gave to her, that was a hard drive containing all of Walter and Tressler's past dealings.
Casual Person