The replica had no heart. It says that. I could see however,if she implanted memories of Riku Replica into sora's memory as the real riku, but Sora has no recollection of what Riku says, so that doesn't make sense. They have two different memory sets. Each of them say they are the one who made the promise to Namine. So that doesn't make sense.
She didn't say he had a heart, but rather that he's a replica of a heart.