My friend is trying to make the case for a change in doctrine by saying that the Church changed its mind on slavery.
I replied that the Church has always thought slavery a bad thing, but not necessarily evil in itself. Slavery could have evil elements, especially its dehumanizing aspects and splitting of families, but there could exist a slavery which, while not the ideal, was nonetheless not against the natural law.
My friend cites the catechism and the encyclical mentioned in this article:
The article has my argument as well.