Who is Ill Friend from The Practice of the Presence of God and what is their importance?

Four letters go to the same reader, a man suffering from an unknown illness. Lawrence advises the man to maintain his relationship with God because of his illness. Such suffering serves as proof of the Almighty's love. Lawrence moves on to admonish the man to examine his own life for something that could hinder his relationship with God, as perhaps the suffering exists for this cause. Finally, Lawrence vows that he would take the man's suffering if possible. However, the lack of such suffering in his own life may mean that Lawrence could not properly bear such a trial.