No, simply signing a card doesn't make someone a Christian. Becoming a Christian involves a commitment -- a commitment of our heart, mind and will to trust Jesus Christ as our Savior and follow Him as our Master.
Only God knows your friend's heart, and whether or not he sincerely believed in Jesus when he was a boy. But Jesus warned us against the dangers of a shallow belief that doesn't make a difference in the way we live. If we truly understand who Jesus is and what He did for us, we won't be satisfied with a shallow faith. Instead, we'll realize that He was the divine Son of God who gave His life for us -- and in response, we'll want to give our lives to Him. As the Bible says, Christ "gave himself for our sins to rescue us from the present evil age" (Galatians 1:4).
Pray for your friend, that he'll face honestly his need to confess his sins and commit his life to Jesus Christ. But I urge you also to look at your own life. Have you ever turned your life over to Christ, and are you seeking to live for Him every day?
Satan will do everything he can to keep us away from Jesus. Sometimes he'll tempt us to completely reject Christ -- but sometimes he'll deceive us into thinking we belong to Christ when we really don't. Don't let this happen to you.