1) Has this atheist never met any Jews, Christians or Muslims who don't believe the Genesis story of the Garden of Eden is literally true, but are still Jews, Christians or Muslims? Really? Really? All of the atheists I've been hanging out with -- have they all recently barely escaped with their lives from believers who were about to slay them because they touched pigskin with their bare hands or ate shrimp? That would explain a few things.
2) If they really are talking to people who believe that the Genesis story of the Garden of Eden is true -- What's to keep those believers from answering, "This painting was made in the 15th century. Why would you expect a painting made then to realistically reproduce every detail of the bodies of people the painter never saw, who lived thousands of years earlier? They've got these buildings full of paintings and sculptures, they're called 'museums,' you should check one out sometime. Paintings and sculptures from all eras, when they depict people and things from earlier eras, are full of historical inaccuracies. There's even a name for such inaccuracies, they're called 'anachronisms.' Painters in the 15th century didn't have access to nearly as much information about the ancient world as we have today, so it's only natural that if they painted someone from ancient history or mythology -- and they made such paintings all the time -- the paintings would contain anachronisms which would be much more obvious to someone in the 21st century than someone in the 15th century. And why should we assume in the first place that all such anachronisms are committed by mistake -- well, we certainly shouldn't! Not only do painters of all eras make unintentional anachronisms -- painters in all eras have made intentional ones too! If the people in the pictures look like people from the viewers' own time and place, it makes it easier for the viewers to relate to them as real human beings. That's Painting Psychology 101, or 201 at most. Movies are chock full of anachronisms too, everybody knows that. Stirrups weren't invented until after the fall of the Roman Empire, but in movie after movie you see ancient Greeks and Romans climbing up onto their horses with the aid of stirrups. You know, I don't know if I've ever heard anyone say that a 15th century painter was a bad painter because there are anachronisms in his pictures. If anybody ever did say such a thing I'd have to disagree, I'd have to say they were missing the whole point of painting! How many times have you heard someone complain because people in a movie who were supposed to be ancient Romans were speaking English and not Latin? And I KNOW I've never heard anybody before this point out an anachronism in a 15th-century painting and say, 'This proves the Bible is wrong.' So thank you, Sir, for a completely new experience and a good laugh!"