How to use a memoir written through another author

How should a historian treat memoirs written by a third party from a subject's recollections?

Use them cautiously and cross-check whether the subject was present, whether context was preserved, and whether other sources corroborate the story.

Mediated memoirs deserve caution rather than dismissal. The first question is whether the person supplying the story could actually have witnessed the events described. The next is whether the author has misunderstood, reshaped, or stripped away the original context. Cross-referencing other evidence is essential before the account can support a historical claim.