All of us, even repentant Christians, struggle with sin. It's in our nature. Part of living in a fallen world. We still sin, even as Christians. The trick is to not be characterized by that sin. In your example that person should repent and ask God to protect them from future temptation. If they are sincere the Lord will be faithful to them. Alternatively, they could ask fellow Christians to hold them accountable. As Christians we are not to judge each other but we are to keep each other accountable. Not to pry but is this a general question or are you dealing with a specific issue?