Let's see -- no Adam, no sin. No original sin, no fall. No fall, no Christ. No Christ, no redemption. No redemption, no Christianity...oops.. However, if you tell a lie long enough, people will think it's the truth. And that is how religion began and grew. That is how we know that all religions are founded on the fear of the many and the frauds of the few. When faith is given credibility as a tool to understand the universe, no flavor of espoused nonsense should come as a surprise though.
What should be done? I agree that Adam and Eve were real but we live in a free country. People are free to think and believe what ever they believe. This include the right to believe something even if you are completely wrong!
Nothing. God will deal with them in His own way and His own time. Its not our business to interfere in free choice given by God. All we are supposed to do is provide Gods word to others. Judgement and belief is Gods responsibility, not ours.
What's sad is when a Atheist wants Christians to argue, have contention, tear down each other... Rather than love and build up one another... Fortunately not all Atheist are this way, But I've noticed the Atheist that posed this question wants this to happen.. I strongly suggest that you tear down your own family... And leave the building up and encouraging to Christians.
I agree that while original sin is indeed spiritually deadly for every person, Christ's work on the cross defeats it. Or, as John Calvin says, "For certainly, Christ is much more powerful to save than Adam was to ruin." People are free to not believe, all we can do is tell them, and love them.
All we can do is to enlighten them about what the Scripture's say. We could go a step further and show them how empirical science agrees with the idea of Adam and Eve. We may not be able to change their mind, and their worldview may be a little strange. But if we show them what the Bible says, and they still don't change their minds, then we've done what we can, the rest is up to God.