Only God knows a person’s heart, and only God knows whether or not your friend was sincere when he made his profession of faith. Sometimes the seed of faith takes a long time to grow in a person’s soul.
At the same time, the Bible does warn us against false or shallow professions of faith—a “commitment” to Christ that isn’t sincere and won’t stand the test of time. Even in Jesus’ day, many said they believed in Him and wanted to follow Him—but when they began to realize what Jesus demanded, the Bible says that “many of his disciples turned back and no longer followed him” (John 6:66).
This is just as true today as it was in Jesus’ time. In one of His parables, Jesus compared the preaching of the Gospel to seed that is sown in a field (see Matthew 13:1-23). Some seed, He said, falls on the pathway and is eaten by the birds. Other seed falls on rocky soil and fails to take root, and still other seed is choked by weeds. But some seed, He said, takes root and flourishes. The same is true whenever the Word of God is preached; not everyone who hears it becomes a true believer.
Pray for your friend—and learn from his example. Make sure of your own commitment to Christ, and don’t let anything turn you away from Him.