Your friend, while spinning it in an anti-Catholic manner, does make some valid points. It is not by divine law that Catholics are required to be married in the Church. It is a matter of Church law, and thus could be changed. For most of the history of the Catholic Church this requirement did not exist, its only about 400 years old. And, as we noted, it applies only to Catholics. Two non-Catholics of any faith or no faith at all validly marry when they exchange consent regardless of where it take place.
The Church’s ability to create laws is found in the Gospels where Jesus gives the apostles the power of binding and loosing (Matthew 16:19; 18:18). The expression “binding and loosing” means forbidding and permitting. Jesus gave to the apostles and their successors the power to create laws that forbid or permit things. One may debate the practicality or reasonableness of some laws, but they remain a valid exercise of authority.
So, why does the Church generally forbid Catholics to be married outside the Church? The answer is pretty simple: respect for the sacrament. All sacraments take place in a church unless there is a compelling reason otherwise. Ordinations, confessions, baptisms, Eucharist, etc. all normally take place in a church. Marriage is equally as important as those sacraments.
With the advent of increasing mixed marriages (Catholics to non-Catholics), this requirement helped protect the faith of the Catholic party and ensured that a proper understanding of marriage was held by both parties.
Historically there were also issues with secret/clandestine marriages. Marriages with no witnesses or very few witnesses were difficult to prove or disprove if one of the parties wanted to leave the marriage. The requirement for a Church marriage made every marriage a matter of official public record.
When it comes to to your friend, the issue at hand is not whether a marriage outside the Church could be valid rather the issue your friend has is with the authority of the Church to make laws.
