How to include the parent page link in this wp_nav_menu walker

Question

WordPress does not have the class Related_Sub_Items_Walker. It looks like you’re using the plugin Advanced Menu Widget.

Judging by the widget’s source, you want to use 0 for the start_depth to include the parent. This is untested.

<nav id="sub-navigation">
  <?php
    wp_nav_menu( array(
      'theme_location' => 'main-navigation',
       'container' => '',
       'walker' => new Related_Sub_Items_Walker(),
       'start_depth' => 0, // 0-indexed
       'include_parent' => 1,
       'strict_sub' => 1,
       'only_related' => 1,
       'filter' => 0,
       'filter_selection' => 0,
    ) );
  ?>
</nav>