If statement for is_author of a post

You can access the posts author (inside the loop) with the global $post;

echo $post->post_author

If your sidebar comes after your main content (in code), then you could populate a global array (not the best solution, but ok).

// in the loop: Add each author to the global $post_authors array
$post_authors[] = $GLOBALS['post']->post_author;

// in your sidebar:
if ( in_array( 'Author Name', $GLOBALS['post_authors'], true ) )
{
    // do stuff
}