Think of the grandmother as being something like a newspaper. She is told some stories about WWI and she records them. Maybe she edits the order in which they were told her, but if she has a good memory, she doesn't change the facts of the story.
That places her in the same category as a newspaper. That's a secondary source. It interviews people to get their stories, but the reporter doesn't repeat the events. He / she merely writes what she has been told up and an editor sees that it is printed. Same with grandmother. She just reports what was told her. She did not re enact the experience.
