You can't change someone else, but you can change how you react to them. If someone you know has backslidden, the best thing you can do is pray for them. Pray that they will come back to God, and pray that they will be given the strength to overcome their temptation. You also need to be mindful of your own attitude towards the person. It's easy to get angry or frustrated with someone who has backslidden, but that won't do anyone any good. Try to stay positive and hopeful, and remember that God is still in control. He can bring good out of even the worst situation.