Or he could at least grow a pair and say, "Yes, the bible was OK with slavery but I think it's as reprehensible now as it was back then, so the bible is wrong on this point." I'd have just a bit of respect for him if he said this. No, he's got to go the "Well, slavery was different back then" cop out. It's a blatant lie that Christians came up with to sooth the cognitive dissonance they feel when learning the bible didn't condemn slavery.
Christian apologetics is the art of rolling a dog turd in sugar and selling it as a donut.