How can one get rid of the debilitating whispers regarding the purpose and benefit of life, and return to a nature full of hope and love for religion, after the death of a friend from cancer?

A person is not held accountable for sadness, but they should not surrender to it. Rather, they should repel it with patience in the face of Allah's decree and destiny, and with the knowledge that what is with Allah is better and more enduring.

The illness that has afflicted the patient is good for them, and it is hoped that it will grant them the status of martyrs.

Among the things that repel sadness are: the knowledge that Allah's decree is unchangeable, and that whoever is content will be granted contentment, and whoever seeks recompense for their affliction will be granted Paradise; striving against the self to perform acts of obedience and to increase in righteous deeds; constant remembrance of Allah Almighty; and diligent supplication.

As for intrusive thoughts (waswasa), they do not harm or affect faith. One must repel them and turn away from them, knowing that Paradise is the abode of complete bliss, and its inhabitants will not experience toil or sorrow therein.