Corey was one of the select few Trainers who actually made it to New Island, arriving by flying on his Pidgeot. He ended up witnessing Mewtwo's plan firsthand alongside Neesha, Fergus, and Ash's group—basically getting caught up in one of the most significant confrontations in Pokémon history.
When things went down, Corey tried to take on Mewtwo's clone Venusaur using his own Venusaur, Bruteroot, but it didn't go well. Mewtwo's version completely outmatched him, sending Bruteroot flying with a single Vine Whip. He also brought his Rhyhorn into the fight, though with similarly rough results.
Like every other Trainer present on the island, all of Corey's Pokémon ended up being cloned by Mewtwo during the conflict.
Interestingly, the character got some odd treatment in localization—in the Latin American and European Portuguese versions of the movie, he was actually renamed Gary Oak and rewritten so the characters knew each other beforehand, with the dub even using Gary's voice actor for the role.
Content compiled by AnimeList.moe from publicly available sources.
