Get new friends.
That would be cruel. His friend is only 6 years old.
I hope this wasn't serious. Politics isn't everything. Besides that we sure won't get any converts if we act this way.
Getting off of my high horse onto the issue, the phrase to describe slavery prior to 1700 is that it was a "human universal." The only place it wasn't found was in some harsh climates, like the artic and the worst deserts, into which the losing sides in primitive warfare were sometimes pushed. What then hapenned? A bunch of British white Protestant men came up with the idea that slavery was wrong -- not just wrong in when it was our own people being enslaved, but wrong in general. Their ideas thankfully have swept over almost all the world.