How do you get the post thumbnail for a relationship item?

Usually you can just use the_post_thumbnail(), but it’s not working for me.

if (current_user_can('manage_options')) { // hide from users until it works
              $companions = CFS()->get('companions');

              foreach ($companions as $post_id) {
                $the_post = get_post($post_id);

                $li = '<li>';
                $li =  ???????;
                $li = '<a title="View ' . $the_post->post_title . '" class="thumb" href="' . get_permalink( $post_id ) . '">' . $the_post->post_title . '</a>';
               $li = '</li>';
              }
echo $li;
            }

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