Normally, players are expected to discard their known-trash cards before discarding their chop.
By discarding her chop instead of the known-trash card, Alice is signaling that something is very wrong — specifically, that Bob's chop is critical or playable.
Bob should permanently move his chop by one position, and write "cm" on his slot 4 to mark it as Chop Moved.
Bob is not allowed to discard on this turn. Instead, he must give a clue.