I thought Kiki physically dying was the entire reason her badge was invalidated? I don't think Flo quitting would be the same situation.
I don't want Fern being the final anything. He's the rival, not the nemesis. I do think his final defeat, whenever it does happen, needs to be as humiliating as possible, with him breaking down in hysterics and cursing you out as he loses. It needs to be his lowest moment and gloriously so.
Way I envision it, once you knock out his pokemon, he doesn't even give you money or even a YOU DEFEATED SWAG JOCK-. The battle just ends after his last mon falls, no victory music either., he starts shaking his head in utter disbelief at his loss, and just starts losing it. He starts calling you all sorts of nasty crap, laments how the family name is tarnished because of you, starts blaming everybody else, starts foaming at the mouth how he's top dog and you're nothing but a sorry cheap ass no-skilled lucky bitch. Then hardy, aya and co. start talking trash to him too, and he's just just like, screaming maniacally and in denial you're better than him. Then during his mad ranting he just collapses in shock blubbering incoherently in an infantile state, face down in his own slobber, vomit, piss and tears.. Like his last defeat needs to break him mentally all the way. It'd be awesome.
And then flobot goes up to him, and embraces him tightly in a loving hug, attempting to cheer up her baby brother. Idk, maybe seeing Fern in such a miserable state will finally trigger her emotions. Like it'd be both sweet and satisfying in a way.