Is it obligatory to continue giving the water on which Quranic verses have been recited to the father who is sick with cancer, knowing that giving him this water causes him to vomit and expel a green substance of unknown cause?

We ask Allah the Almighty to cure your father, and we advise you to supplicate frequently and not to despair. There is no harm in continuing to give him water over which ruqyah has been performed, if the ruqyah is legitimate, as the vomiting that occurs might be due to the effect of the ruqyah, and what comes out might be a harmful substance that is the cause of his illness. Al-Bukhari and Muslim reported that a man came to the Prophet, peace and blessings of Allah be upon him, complaining about his brother's stomach, so he commanded him to give him honey to drink, and he was cured. If it becomes clear that drinking the water harms him, you can sprinkle him with water over which Quran has been recited instead of having him drink it.