Monte,
You are correct that the 7th day Sabbath is ceremonial, but Adventists look at it differently. They hold to a dual law theory. The dual law theory in a nutshell is that the 10 commandments are God’s moral law and is unchanging, but that everything else written in the books of Moses are ceremonial in nature and not now necessary to keep.
This position is relatively easy to overcome. First, the dual law theory is not biblical. There was only one “book of the law” and it contained both moral and ceremonial aspects to the law. One only has to read some OT passages, let us say, like in Leviticus, to see that there is much morality taught outside the 10 Commandments. And they would not argue that these are to be discarded. Additionally, in practice, they also hold to the Jewish dietary laws, and are strict tithers, both of which are not contained in the 10 commandments.
Another way to overcome the argument is to have them really read Acts 15, which contains the first counsel of the Church. It is clear here, that the gentiles were never required to keep Jewish laws, and that was the decision of the Christian Church with the guidance of the Holy Spirit.
You could also point to The Apostolic Fathers, like Ignatius of Antioch in his letter to the Magnesians, where he states we are not to observe the 7th day Sabbath, as the Jews do. Your friend may dismiss this as an extra-biblical source, but it is none the less valid. Remind him that the letter was written in the first century, and that Ignatius lived in the time of Christ, was a disciple of the Apostle John, and appointed by Peter as Bishop of Antioch.
I hope this at least gets you started.
Peace,
apruett