You mean survived the graveyard, right? Not that he just let Harry take the goblet and stay in the labyrinth.
Well, I think people would believe Harry less about Voldy being back, because yes you have a second witness but no reason to make people believe something went wrong (no corpse). Cedric would have supported Harry through and through though.
He probably would have broken up with Cho (It's not clear in the books, but I always get a vibe from her in the GoF movie, that she liked Harry before).
He would totally become somewhat of a brother to Harry, after all they've escaped death together.
Nightmares on both sides, so Cedric probably visits privet drive in the summer and you know check on his little friend.
Cedric would have tried to get the school to not split up into Slytherins vs. the rest.
He would feel for the first time what it's like to be unpopular though, since the daily prophet wrote some smack about him too.
But his Hufflefriends always have his back.
He starts being more around the Weasley twins, at some point telling them about how both he and Harry feel after what happened. This results in them developing all sorts of knick knacks for them like good dream charms where you can choose what to drem, anti angst drops, making you calm the longer you have them in your mouth or a cheer-up button that just cheers the one wearing it up whenever needed.
If he doesn't die in the battle of Hogwarts, he helps George cope a lot since, he's always been the best at Hufflehugs. He's the first to forgive Draco because "He's just a kid, he's still a baby, just like you" and then he's horrified about sounding like a mother hen. But accepts it pretty fast and runs around protecting his baby brothers because he knows what's out there, and he couldn't help Harry the first time, he'll be damned if that ever happens again.
But most importantly: The cursed child never happens
