After they finished shopping and got back to Jon's house, Karyn grabbed the wishing stone when Jon wasn't around. After thinking about it she had decided that she didn't want Jon to become a boy. She wanted her to stay as a girl, because that's how Karyn always remembered her.
So, after grabbing the stone, she said "I wish that the effects of the big red button device would never wear off." The stone glowed, granting her wish.
Then Jon came back into the room. "Did you say something, Karyn?"
"Nope," she answered, making sure Jon didn't see her holding the wishing stone. She might not like it if she knew Karyn was making wishes behind her back.