Only God knows your friend's heart, and whether or not he had truly believed in Jesus and given his life to Him. But the Bible also warns us against false belief -- thinking we have turned to God, when in reality we haven't.
How can this happen? Jesus warned us that sometimes people seemingly welcome the Word -- but in reality their hearts are so hardened to God that the Gospel can't take root and grow. The Word of God, He said, is like the seed a farmer scatters -- but sometimes the seed lands on soil that is too hard or rocky for it to grow: "Those (seeds) on the rock are the ones who receive the word with joy when they hear it, but they have no root. ... In the time of testing they fall away" (Luke 8:13).
Pray for your friend, that he will remember how good God was to him during a very dangerous time. Then pray that he will turn to Christ for the forgiveness and new life he needs. He may think giving his life to Jesus is a sign of weakness -- but it isn't. It takes courage to follow Him.
Then examine your own life, and ask God for a heart that is open and ready to receive His Word -- not just when you first believe, but every day of your life.