Most systems that we work with here in southwest PA have a "comments" field which allows for the notation of corrections to the index.  However, some do not.  The most simple solution in such a case is to have your searcher make a copy of the index both before and after the correction is made.  That way, if an issue ever arises, you have hard copy evidence that the item was mis-indexed.

On the other hand, if you can find the error, why can't the other searcher?

Regards,
Scott Perry