Which is better in God's eyes: to do what is right (and lose a few dollars), or to do what is wrong (and save a few dollars)? I think you know the answer.
In other words, make your decision based on what God wants you to do, not on what might cost you less money. Living together without the commitment of marriage may be popular today but it still is a violation of God's plan for a man and woman regardless of your age. God's pattern for marriage is clear, and has been so since the beginning of the human race: "For this reason a man will leave his father and mother and be united to his wife" (Genesis 2:24).
But I also think you need to face honestly the question of whether or not this is your friend's real problem. After all, if one of you gives up your cottage, won't your overall expenses be quite a bit less? To put it bluntly, is he hoping to have all the advantages of marriage without the commitment, and without the legal responsibilities? Don't get involved in something you will only regret later.
The most important thing you need to do, however, is to make Christ the center of your life. He loves you, and He knows what is best for you both now and eternally. Don't be satisfied with anything less than Christ and His will for your life.
Follow the Steps to Peace online to learn about Jesus or recommit your life to Him.