To add to the other replies, my own grandparents sound similar to yours in that they were happily married for 52 years. But my grandfather passed away 5 years ago, and while I know she still misses him deeply, I don't think my grandmother is in a constant state of misery. She is still able to find happiness in life despite now being alone.

I think that's what the parent was getting at. Being devoted to your partner doesn't mean you need them to enjoy life.