Marianne,
I am a cancer survivor and early on I felt much the same as your dad does now. I was ultimately convinced that Jesus was not a quitter and that God would call me home when the time was right and that, in the mean time, it was necessary to do everything possible extend my stay as I and all of us were put here for a reason --- to do as much good as possible during our brief visit here. We have all spent a lot of effort trying to keep ourselves safe and sound, and treatments are just another means of doing so, no different than other more routine precuations.
I'd suggest that you have a very firm talk with the cleargyman who gave your father that message, inform him of the grave and deep impact it had on him, and ask him to visit and explain to your father that he must do everything possible to extend his stay on earth so he can help make a better world through his trials of life and wisdom. That God would expect him to. Perhaps the clergyman said something and his comment was mis-interpreted by your father. During difficult and trying times that can easily happen. Unless the clergy was a trained and professional cancer doctor, I equate his comment, if correct, akin to manslaughter -- a grevious sin.
My prayers are with you, and your father.
Milt