['It was really tough to watch my father struggle with cancer. When he got the diagnosys I was devastated for him.', 'I am sure that was difficult.  How is he now?', 'He passed away. At least he is no longer in pain.']
I am so sorry for your loss.  It can be a relief to know he isn't in pain.  I felt the same way when I lost my Aunt last year.  She was like a mom to me.
When my father was diagnosed with cancer, I was so devastated for him. It broke my heart to see him in pain.