Prevent author from changing their posts if admin has modified

You can try the following:

/**
 * Post Update Locker For Authors
 * If an administrator has updated the post, then lock it for author updates.
 * @see http://wordpress.stackexchange.com/a/168578/26350
 */

add_action( 'pre_post_update', function( $post_ID, $data ) {

    // Target only authors:
    if( ! current_user_can( 'edit_post' ) || current_user_can( 'edit_others_posts' ) )      
        return;

    // Target only 'post' post types:
    if( get_post_type( $post_ID ) !== 'post' )
        return;

    // Fetch all administrators:
    $admins_ids = get_users( 
        array( 
            'role'   => 'administrator', 
            'fields' => 'ID' 
        ) 
    );

    // or hardcoded if needed:
    // $admins_ids = array( 1 );

    // Check if administrators have modified the current post, by checking the revisions:
    $posts = get_posts( 
        array( 
            'no_found_rows'             => true,
            'update_post_meta_cache'    => false,
            'update_post_term_cache'    => false,
            'posts_per_page'            => -1, 
            'fields'                    => 'ids', 
            'post_parent'               => $post_ID, 
            'post_type'                 => 'revision', 
            'post_status'               => 'any', 
            'author__in '               => $admin_ids,  
        ) 
    );

    // Halt if an administrator has modified the post:
    if( count( $posts ) > 0 )
        wp_die( __( "Sorry, you can't modify this post, it's already been modified by an administrator! " ) );

}, 10, 2 );

You might have to adjust this and test further.

Update:

Here’s an additional example how one can remove the submitdiv metabox for authors that can’t update the current post anymore:

/**
 * Hide the publish metabox for authors if an administrator has updated the post.
 * @see http://wordpress.stackexchange.com/a/168578/26350
 */

add_action( 'admin_menu', function() {
    add_action( 'load-post.php', 'wpse_author_update_locking' );
});

function wpse_author_update_locking()
{
    // Setup - Modify this to your needs:
    $admin_ids = array( 1 );
    $cpt="post";

    // User input:
    $_pid    = filter_input( INPUT_GET, 'post',   FILTER_SANITIZE_NUMBER_INT );

    // Target only $cpt post types:
    if( get_post_type( $_pid ) !== $cpt )
        return;

    // Target only authors:
    if( ! current_user_can( 'edit_post' ) || current_user_can( 'edit_others_posts' ) )      
        return;

    if( $_pid > 0 )
    {
        // Check if administrators have modified the current post, 
        // by checking the revisions:
        $posts = get_posts( 
            array( 
                'no_found_rows'             => true,
                'update_post_meta_cache'    => false,
                'update_post_term_cache'    => false,
                'posts_per_page'            => -1, 
                'fields'                    => 'ids', 
                'post_parent'               => $_pid, 
                'post_type'                 => 'revision', 
                'post_status'               => 'any', 
                'author__in '               => $admin_ids,  
            ) 
        );

        // Halt if an administrator has modified the post:
        if( count( $posts ) > 0 )
        {
            remove_meta_box( 'submitdiv', $cpt , 'side' ); 
            add_meta_box( 
                'submitdivmod', 
                __( 'Publish' ), 
                'wpse_post_submit_meta_box', 
                $cpt, 
                'side', 
                'core' 
            ); 
        }
    }
}

function wpse_post_submit_meta_box()
{
    _e("Sorry, you can't modify this post, it's already been modified by an administrator!");
}

and then instead of this metabox:

the author sees this: