Try the bible. It shows quite clearly the fallen nature of Human Beings, expressed in this case their almost infinite capacity for denial and compartmentalisation.
I agree but did you notice the part about he’s a secular humanist?!
That isn’t the reason the US allowed slavery at its inception
The Bible also does not treat slavery as the "unmitigated crime to end all crimes" as does secular humanism. There are regulations in the Bible governing the administration of slavery. There are no regulations governing the administration of murder, theft, false witness, adultery, rape or homosexuality.
Start with discussing indentured servitude with your friend. Then do a detailed comparison with the terms and conditions of enlistment in the combat arms of the US military during wartime. Then for draftees during war.
Compare that with slavery.