The question "Why has God allowed this?" is a dangerous one, for it arises from a heart that seeks to understand what God has chosen to veil in mystery. The dispensation of God is received by faith, not by inquiring into reasons. When we ask such things, we set ourselves up as judges over the inscrutable wisdom of the Almighty, forgetting that we are but clay in the hands of the potter. Did not the Apostle teach us that we should not trust in ourselves, but in God who raises the dead? This affliction is permitted not to cast you down, but to teach you to rely wholly on Him. When the mind is overwhelmed, it is then that God desires to dwell in it, if only we offer Him a broken and humble heart. Cease from questioning; instead, cry out with the Psalmist, "I was silent and opened not my mouth, for it was Your doing." This silence before God is the mother of sound judgment.
You mention the power of prayer and hold fast to the possibility of healing. Indeed, all things are possible to him who believes. You have heard how the Lord often said, "According to your faith be it unto you." He did not require perfect understanding, but a firm and unwavering trust. The woman of Canaan pleaded no merits, argued no justice, but simply clung to His mercy, and her daughter was delivered. So too, let this man, your friend, not waver through unbelief but press forward in bold, humble faith. Yet, let him also learn from the great cloud of witnesses that true healing does not always come in the dispelling of bodily sickness. What is more wondrous, to have a body made whole for a few fleeting years, or to have a soul refined like gold, made fit for the eternal bridal chamber, where the voice of the Bridegroom is heard and joy is full? Sometimes the Great Physician grants a deeper cure, severing our attachments to this life that we may long for the incorruptible.
Therefore, let your prayers be persistent and full of tears, but let them be offered in submission. Ask not only for the removal of this cancer, but most earnestly that his faith fail not, that he be not ashamed of his hope, and that whether in life or in death, Christ may be magnified in his body. The power of prayer is amazing indeed, because it aligns our weak human will with the loving, perfect will of God. Let this trial not minister questions, but strengthen the dispensation of God in faith. Stand with him in this battle, not murmuring against the storm, but anchoring his soul in the haven of God's unchangeable goodness.