Considering how Karyn reacted after the first time when Jon turned her into a ditz, he was fairly confident that he had already taken things too far.
If he were to revert Karyn now it would take forever for her to forgive him, if ever. And even if she did, could he really trust her? She could just be pretending so she can get her hands on either the device or the stone and exact her revenge.
"In otherwords I'm already at the point where I'd need to use the stone to keep her from hating me" thinks Jon "if I'm already at that point, I can't make things worse... Might as well enjoy ourselves for now."
"Sounds like a wonderful idea lead the way."
