He decided to... Give him what he wished for.
Jon grinned to himself, it was genius. It granted his wish of being in Biff's place and it screwed with both of them!
Jon found a wish granting app on his hosting and loaded it up. He found that what ever he typed there was granted but also that he could changed the details easily at a later date. He began to type.
"Steve will wake up tomorrow morning and find himself in the ultra sexualised body of the hot girl Biff wishes he was. She will experience all the things that Biff fantasies about, she will retain her mind and memories but will be unable to stop herself from being a slut. Finally, everyone else will think that she has always been a girl except Biff who will know instinctively that she is really Steve and that she has had the wish granted instead of him."
Jon opened a viewing window, sat back, and watched the show unfold...