That answer was...
First of all, the original wish had been for Jon to become a girl until he and Karyn get back to her house that night. That night! It was already the next day. Jennifer couldn't change back to Jon by visiting Karyn's house any more unless she figured out a way to fix that.
Second, there was a way to fix it. But Jon hadn't asked the stone to tell him the answer, just to tell him whether there was a problem.
Third, if Jennifer did figure out some way to change back to Jon, she could do it. There would just be some reason for Jon to still have a gynecological appointment despite being a boy. All the reasons why Jennifer could imagine a boy having a gyecological appointment weren't very good, and she decided she might be better off staying as a girl until then, even if she could figure out a way to change back....
