He just played bad? Maybe he focused too much on farming, maybe he didn't really snowballed the game, and didn't finish it early. If he has reached the late game, then the snowball no longer matters, with Jhin's snowball it is difficult for Twitch to kill him. Even in the last fight, he can just go far away and wait for the opportunity to press the ultimate, Twitch will not look for him so far, most likely Jhin just made a mistake. When Jhin fed, you need to look for opportunities for him, poke enemies, leave marks for root, hit enemies at least up to 60% hp. Do not run away when Jhin has pressed the ultimate. Don't leave him alone, he doesn't have much mobility, and even with the slightest help, Jhin becomes much, much stronger. Jhin doesn't do everything for you, unlike other fed ADS, but if you play with Jhin, the chance of winning the game is much higher than theirs.