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.
Check out Avenue 5, hearing him switch between accents constantly is odd but fun.