He was wrong over many accounts. First, the Catechism of the Catholic Church notes the eternal Covenants God has with the Jewish people--and if I recall correctly, with Muslims as well. It notes that non-Catholic Christians are still members of the Body of Christ. Only a very small fringe of Catholics see Mary as co-redeemtrix/mediatrix, apparently based on the fact Mary was his mother. It notes that the Church is still less powerful than God, and those whom the Church cannot reach, it is possible that God does.
It appears your sparring partner may have missed some major Catholic teachings.