Normally it's simply 'lui' = 'he' and 'lei' = 'she'. However, 'lei' is also used as a polite form of address (often capitalized then), so in your dialogue it means 'you', not 'she': B: ... and you? ... A: ... to you too! 'Lui' isn't used that way, so you wouldn't put it in that sentence. "Buona giornata anche a lui" would mean "a nice day to him too".

lei = she lui = he Lei = you formal It can be confusing, but once you get the hang of it, it becomes easy. However, in that dialogue, I believe you would use “tu” instead of “Lei” because of the usages of all other informal words. With the context of “ciao”, which is informal, and “stai”, which is also informal, I think the correct word would be “tu” so that the dialogue would be: A: Ciao. Come stai? B: Sto bene, grazie, e tu? A: Sto molto bene. Grazie. B: Buona giornata! A: Buona giornata anche tu!