Just thought about something fun.
In the latest XY round that came out in Japan, Diantha saves herself from being potentially decapitated by hiding her Gardevoir’s Pokeball under her foot. She explains that a friend told her about a ‘remarkable trainer’ who used the same move. She’s likely referring to Red during his battle against Giovanni at the Viridian Gym.
So basically, back when Green and Diantha were studying at the Tower of Mastery together, he must’ve been constantly bragging about his one true rival to her. Because if he told her that story, there’s no way he didn’t mention all the other stuff he and Red actually did together during the RGB chapter.