In Chapter 32, Law speaking up for Mar when she's all but convinced herself that her existance was a mistake that was never meant to happen
"That woman made many, many mistakes. But you, Mar, are not one of them."
A note by the narrator, in the final chapter.
"The reason I'm telling this story to you all is to fulfill a promise made to Mar, and let you know of her name and deeds so it will not be forgotten how, once, in a reality that no longer exists, she lived."
It is also strongly implied that the Lemony Narrator is actually some part of Disinfector!Law's consciousness that was retconned out in The Wish, meaning he did get to keep his promise to Mar after all.