If you write a memoir by using only your brain, in other word, your memory, it world be really challenge or very short. I don’t believe you can remember all things that has happened in your life. For example, did Solomon Northup really remember all days of twelve years? I don’t think so.
Anyway, I think that there are situations in which a writer tries to capture a story that he or she was not there to witness too. When writers are faced with the problem, they should ask the people who were there and research for it. However, I am not sure wether it is still a “memoir” if you use other’s voice. That’s why writing a memoir is harder than writing a fiction.
Even the memoir that is nominated as a bestseller can be fake. In class, we read A Million Little Pieces which was not even a memoir at all. The readers used to believe that it is a memoir. Therefore, I think that a writer is better not to capture a story that he or she was not there to witness.