I liked to play where if my players tricked me, well they got me, we'll adjust and keep going. They always realized that I may ask a similar question to them though, and it could always go the same way.

In this specific case, I'd let it happen and they'd probably going on an adventure for a wish in the next session (depending on how important the NPC is to the story, they might need to have him as a humanoid). Just like if you have an asshole paladin, they might find themself trying to atone to get back their favor with the gods.