figured it out!
there's a room in the pokemon center in foreplay town, accessed by going up the stairs inside, where you can change your outfit. you have to get the silk nightwear, go there, change into the associated outfit, and then come back and interact with the ariados.
honestly i think if an "equip" and "unequip" action were able to be added to this and other outfits in the future it'd be more intuitive.