Fascinating that this is even a question!
In what way, and against whom, would you be committing a sin?
Two people have chosen to marry. They have not had a priest perform the marriage. It is a valid and legally binding union according to our government.
“Let every soul be subject to the governing authorities. For there is no authority except from God, and the authorities that exist are appointed by God. Therefore whoever resists the authority resists the ordinance of God, and those who resist will bring judgment on themselves. For rulers are not a terror to good works, but to evil. Do you want to be unafraid of the authority? Do what is good, and you will have praise from the same. For he is God’s minister to you for good. But if you do evil, be afraid; for he does not bear the sword in vain; for he is God’s minister, an avenger to execute wrath on him who practices evil. Therefore you must be subject, not only because of wrath but also for conscience’ sake. For because of this you also pay taxes, for they are God’s ministers attending continually to this very thing. Render therefore to all their due: taxes to whom taxes are due, customs to whom customs, fear to whom fear, honor to whom honor.”
Romans 13:1-7
Since this is a marriage between a man and a woman, and if all evidence leads you to believe that they have married for valid reasons and intend it to be a lifelong bond, then you should honor it. One of the ways you can honor their marriage is to bring it before the Lord in prayer, asking Him to place it upon their hearts to have the marriage blessed by the church. If this man is your friend, then you owe it to him to bring this matter before him in a loving manner as well.
I wish you the best.
Jake