It is liberating when a Muslim friend realizes that God accepts us simply by faith, not by our religious deeds. Your friend’s starting position is typically that God at the last day will balance our good works against our bad, and there is no hope if your “bad” outweighs the good except the generalized assertion that “God is merciful.” Two tracks are needed to change this view. The first is negative: facing the fact that humans never can and never will reach God’s standards. Your friend can never be sure that the good in his life will outweigh the bad. The second is to see that the mercy of God, which your friend already accepts, logically would lead God to provide a way for someone else to take the penalty for your sin.
I usually present these insights in terms of my own life. As a grow in faith, I see more and more the depths of my disobedience. It is not that I become better and better, and so need God’s help less, but just the opposite: the more I progress in faith, the more clearly I see how I have disobeyed God in thoughts, words, and actions. But if God is truly merciful, I conclude that God will therefore be merciful to me, and I have accepted God’s promise that God sent Jesus to receive the penalty for my sin when He died on the cross.