Is it permissible to inform my father, who has cancer, of his illness, knowing that he is unaware of it and does not pray, so that he may return to his prayers, fulfill his debts, or make a will?

Reminding a Muslim of death is essential, as the Prophet, peace and blessings be upon him, said: "Remember often the destroyer of pleasures [i.e., death]." It is not necessary to inform an ailing parent of their illness if doing so would negatively affect their psychological state, unless it is deemed necessary as a means for their repentance. They should be reminded of death and repentance through tapes and messages, and you should ask one of the righteous to visit them to offer advice. Encourage them to pray, as prayer is not waived for a sane person due to illness. You can read parts of "Riyadh as-Salihin" and "Sahih at-Targhib wa at-Tarhib" to them, with gentleness and kindness, for Allah Almighty says: "And your Lord has decreed that you shall not worship except Him, and to parents, good treatment. Whether one or both of them reach old age [while] with you, say not to them [even] 'uff,' nor repulse them; but speak to them a noble word. And lower to them the wing of humility out of mercy."