One of the problems with Protestantism is there is no single authority, everyone (seemingly) interprets the Scriptures as he feels he is moved by the Spirit. This ends up in a tangled mess of different doctrines. With the SDA movement which is founded more on the writings and “visions” of Ellen G White, you compound the problem.
It is clear in scripture that God revealed in Acts that the Law of Moses was abolished, the sheet came down from heaven with animals that had been unclean under the Law of Moses. “What God has made clean, let no man call unclean.” Our Lord Himself spoke of a New Covenant in His Blood, the Covenant with Moses and the Children of Israel was supplanted by the New Covenant. So too was the Covenant with Abraham supplanted by the Mosaic Covenant. The Apostles, where given authority to apply and interpret the New Covenant under the influence of the Holy Ghost.
The problem with SDA’s is they historically are not Trinitarian, in denying the divinity of Christ Jesus, He becomes just another, albeit the greatest of the Prophets. By relegating to Him humanity and denying His divinity, the Authority that Ellen G. speaks is just as valid and just as authoitative as what Jesus and the Apostles said. By sticking to the Protestant and very anti-catholic misrepresentation of history they can ignore the historical facts that prior to the timeframe that they say the Catholic Church was formed as a blending of Christianity and Paganism, the Church observed Sunday as the Lord’s day. It is funny that they will grant to Ellen a broader infallibility than the Catholic Church limits to the office of the Papacy.
I would ask your girlfriend’s brother if he believes that all men should be circumcised, and that an animal should be sacrificed after their birth? After all the Apostles and the first converts continued to attend the synagoge and Temple services at first, and prior to the Council of Jerusalem most required circumcision before baptism. Either the Church has the authority to change the day of worship, and other requirements for entrance into the Church, and for worship, (within limits) of it does not. As it was Jesus who gave the power to govern the Church to Peter, and not the Church who simply ratified what they wanted to receive from Christ, I’ll stick with what the Apostles said, and handed down (traditio) to the Church.