Ask questions to deconstruct their worldview first.
For example, if they attack Christianity as nonsense, ask them: What's wrong with learning non-sense from an atheistic WORLDVIEW? On atheism, why is knowledge important? How does an atheist gain knowledge (as a product of evolution)? Why, as product of evolution, should you trust your senses? What in the primordial soup (and on) makes logic 'work'? If your'e just a product in a chain of animals, you have no choice but to obey your brain! As a mere animal, the atheist is a slave of their genes. So how can they guarantee their 'brain fizz' is actually logical and not just chemical reactions? The same exact thing goes with morality.
What, in evolution's long-line-of-tinkering created morals? How is that a material product in any way? It's not. What is the source of morality? What is truth in a materialistic world? And why is truth important - some lies may help people survive. And isn't that the point of evolution? Where did truth come from? Do plants have this concept? Do worms? Truth is an abstract concept. Ask him if he believes that things that are immaterial like morals) are actually real. How does he account for an immaterial concepts like morals in a materialistic worldview?
Are his morals his own or does he think others should follow him? Maybe the atheist could give an example of an example of how his morals differ from our current culture - and prove it. For example, many atheists are for same-sex marriage. Would their morals have told him that 100 years ago? Or would they have just went with the current culture then, too?
This means from their own view of what they are, the atheist has no business worrying about slavery. Survival of the fittest, right?
Some atheists say that 'morals are values that correspond to goals'. That's not morality, that's crass pragmatism or utilitarianism. It's just a word to describe strategy! Besides, who decides if the goals are good or moral? That definition is meaningless because the atheist has no foundation on atheism for objective morals - or even the whole concept of morality. Remember to be witnessing while debating. We are trying to get the atheist to see their sin and turn to Jesus (I am not saying I always do this)! Many love to debate, but they need to be saved.