If a writer does not remember or did not witness something for his memoir it is very important to not make things up. Just because you do not remember it does not give you an opportunity to write whatever you want that will make your story better. The writer needs to ask around and stay as close to the truth as possible. It does not have to be identical but the main idea has to be the same. As long as just a couple parts of the story are like this your memoir can still be a non-fiction memoir, however, do not do this many times throughout the book or your memoir cannot be published as a memoir.