I am so sorry for your suffering. My mom died almost 2 months ago from a 2 year struggle with pancreatic cancer. She was at hospice in the end and they really are a wonderful organization. I think her being there gave my dad some rest and therefore energy; and the setting to be with her in her final days in a more emotional spiritual way since he did not have to tend to the medical part.
We prepared for 2 years for her eventual death and although at first I had found peace in the end to her suffering and how peacefully she died, I now struggle with deeply missing her. It is still surreal. We were very close and my parents were also married 51 years. I think it was important that we gave her permission to be done. It is hard and there is no way to prepare really. My faith is what keeps me going. My parents are 10 years older than yours so in a way my dad is more frail...in that physically it is difficult for him to try and stay busy to help ease the pain if loss. He (and we) are just very sad about our loss. I pray for you. It is painful and it is difficult to go from trying so hard to keep someone alive to switch to letting them die. I would say she needs your dad from more of an emotional (saying goodbye, calming her fear) than he needs her physical care BUT he needs to have peace with whatever decisions he makes so that he can have peace after she is gone. regret can eat at a person.
All my best to you and your dad