you have to increase relationship with Mays mother to the point she asks you to remind the professor of their date plans, then go see the professor. he will talk about a notebook he lost blah blah blah.
the notebook is above rustboro city (where you can battle cirno)
the notebook is being guarded by the Vulpix. defeat her to add her to your pokeslave dungeon lol. then take the notebook back to the professor and hell ask you to fill in on the date, which earns you a Paizuri scene with Mays mother