Mary, I am so sorry to hear your news, Difficult question! Have you asked your dad what he wants?
I guess a dilemma like this begs so many questions - What will the side effects of the radiotherapy be? How much suffering will he endure in the 'bought' 6 months - and what will his quality of life in those 'bought 6 months be?
I couldn't possibly answer your question, but am here as a friend in support, if you need me! I am sure that you know in your heart that any decision made will made for what you believe will be the right reasons and therefore will be the right decison. Sending you, your dad and family much love and prayers that you all get through it without too much suffering.
Much Love, Lucy