My wife ran into an old friend at a store, and they were talking. Her friend talked of how her and her fiance were living together, and my wife said something that kind of approved it. Her friend was also cussing. I walked away and sat in the car. I'm afraid because I didn't say anything I approved of this girl's sin. Should my wife go to her and let her know she doesn't approve of her fornication? Should I go to her and let her know I don't approve of her fornication? Or should we just confess it to God and do better in the future?
You did show some disapproval by walking away, but not in such a way that this other woman would understand what was wrong.
The problem is that you both lost an opportunity to tell someone that God's way works better. A simple question, such as "Do you really enjoy living with a guy who hasn't made a commitment to you?" could lead to some interesting discussions. Or she might just walk off in a huff. But look for opportunities to get people to think about their sins and see that what they have been ignoring really does matter.
So apologize to God that you didn't have the courage to use the opportunity that He gave you and ask for another opportunity to shine His light. "Do all things without complaining and disputing, that you may become blameless and harmless, children of God without fault in the midst of a crooked and perverse generation, among whom you shine as lights in the world, holding fast the word of life, so that I may rejoice in the day of Christ that I have not run in vain or labored in vain" (Philippians 2:14-16).