I had a coworker make that point to me, but in a gentler way, when my grandma was dying. I was upset about it and she said "Well one way to look at it is that she won't be suffering anymore." She told me of how her grandfather had Parkinson's, another one of those illnesses where you just get worse as time goes on. She said it was a relief to see him go.
It helped but again, she phrased it better than that guy did.