Question: "How do I pray for someone if I'm not sure about his standing with God? He says he's a Christian, but I'm not sure." Answer: Only God knows the heart. If you can't see the fruit of someone's faith in Christ, that person is likely far from God. Don't let your confusion about someone's salvation keep you from praying.
Instead of getting caught up in trying to read someone else's heart, focus your prayers on what you know: his fruit (or lack of fruit). Let's reframe the question: How would you pray for someone who is far from God?
An example: "Heavenly Father, you and only you know the heart of man. Your word says that the heart is deceitful, yet nothing escapes your understanding. I intercede on behalf of my friend. I believe that he is far from you. I pray that you will do whatever it takes to draw him to yourself. Let me be a faithful witness and a loving friend. I pray that he will see Christ in me. Glorify your name in all that you do. Amen."