What does "Naughty" mean in this context?
John: 'And as a special thank you for all you've done, we're treating you to a meal out.'Sue: 'You're really naughty.'

Question

Answer

In that context in British English it means 'oh you shouldn't have!' (Treated me to a meal)...'you're so cheeky!' 
It's common to (playfully) pretend to turn down someone paying the whole tab & then accept the other person's treat.

Answer

Sue means that John is doing something she thinks he shouldn't be / isn't meant to / doesn't need to. 
It's a colloquial way to thank you and that their action is unnecessary.

No pierdas la oportunidad de aprender un idioma desde la comodidad de tu casa. ¡Explora nuestra selección de profesores de idiomas con experiencia e inscríbete ya en tu primera clase!