Castelia City
She is located in the first building on the left side of Castelia Street.
Castelia Street has many people coming and going, especially during the evenings when people are going home from work. This street has many tall skyscrapers and large buildings; however, it features the Game Freak HQ and a building where one Pokémon per day can receive a massage to increase its friendship.
Source
Team AB.
&
Team Swagg.