How would you react to someone who says that thank you notes are unnecessary?

A woman I know was recently discussing her upcoming wedding anniversary party. She mentioned that a relative of hers was not attending the party because they were still angry that she did not send them a thank you note for the wedding gift they gave her. She described the gift and it was extremely generous. She was married two years ago. She said that she thanked everyone in person or by email and she felt that should be enough. She added that “besides, I’m saving a tree.” She really seemed put out that someone expected her to write a thank you note. I was floored, but chose not to say anything as she would not have liked my opinion.

What do you think about this? If you were me, what would you have said? If you were the relative, how would you have reacted?