My cousin told me she believes that Jesus ceased to be human after ascending into heaven. This sounds to me like a heresy. Has this heresy appeared in history and has the Catholic Church officially condemned it?
Jesus did not cease to be human. Jesus is fully human and divine. His human body was resurrected and glorified. At the Ascension He ascended into heaven in His glorified human body.
Catechism of the Catholic Church (emphasis added):
659 “So then the Lord Jesus, after he had spoken to them, was taken up into heaven, and sat down at the right hand of God.” Christ’s body was glorified at the moment of his Resurrection, as proved by the new and supernatural properties it subsequently and permanently enjoys. But during the forty days when he eats and drinks familiarly with his disciples and teaches them about the kingdom, his glory remains veiled under the appearance of ordinary humanity. Jesus’ final apparition ends with the irreversible entry of his humanity into divine glory, symbolized by the cloud and by heaven, where he is seated from that time forward at God’s right hand. Only in a wholly exceptional and unique way would Jesus show himself to Paul “as to one untimely born”, in a last apparition that established him as an apostle.