In Ella Enchanted, Prince Charmont is visiting the court in Ayortha during Ella's stint at Finishing School. And Areida is there at Finishing School at the same time. And during this time Aza is there at court too. But we never hear mention of the Prince at all during the course of the book.
It's possible that he avoids the Ayorthains as much as he can since they seem to make him uncomfortable.
Also, the exact timelines of the two books with respect to each other are never fully established. It may simply be that Charmont's visit didn't take place until after the end of Fairest.
It's also possible that Charmont's time in the Ayorthain court was during the months Aza was living with the gnomes, which would explain why, in Charmont's letters to Ella, he implies the courtiers are hesitant to speak at all — feelings towards Kyrria, Ivi's country of origin, would have been tense at best during this time.
I think it’s implied that Char left before the wedding and all the attendant drama. Areida leaves for finishing school “a few months” after Aza discovers illusing at age fourteen, and Aza has her sixteenth birthday shortly after Oscaro and Ivi’s wedding. That could easily leave more than a year between Ella meeting Areida at finishing school and Aza’s wedding, so that checks out in terms of the length of Char’s stay. It might be a bit weird, since royals do usually go to each others' weddings, but not nearly as weird as Areida neglecting to mention that her sister is married to the Ayorthaian crown prince (if it was after the events of Fairest) or Char cheerfully writing letters about imports and exports while civil unrest brewed in the background (if his time at the Ayorthaian court overlapped with Aza's).
Aza's birthday actually happens while she's staying with the gnomes, meaning Char probably would've had to leave a few weeks earlier than that. Still, depending on exactly when Aza's birthday is, this could still leave just enough time for Char's visit to happen sometime during the book. It's also possible that Char was indeed present for the wedding and Aza just didn't notice him, given that she was trying her hardest not to call attention to herself at the time.
