Ah, that makes sense. So this is rather a reverse relationship query. In that case your friend was correct, but it can be done in a single query:
<?php
$related_posts = get_posts(array(
'post_type' => 'post',
'numposts' => -1,
'post_status' => 'publish',
'orderby' => 'title',
'order' => 'ASC',
'meta_query' => array(
array(
'key' => 'related_products',
'value' => '"' . get_the_ID() . '"',
'compare' => 'LIKE'
),
)
));
?>
<?php if ( $related_posts ): ?>
<ul class="medium-block-grid-5 blog-posts">
<?php foreach( $related_posts as $rp ): ?>
<li class="post">
<a href="<?php echo get_permalink( $rp->ID ); ?>" title="<?php the_title_attribute( array( 'post' => $rp->ID ) ); ?>" ><span class="featured-title"><h3><?php echo get_the_title( $rp->ID ); ?></h3></span></a>
<?php if ( has_post_thumbnail( $rp->ID ) ) : ?>
<?php echo get_the_post_thumbnail( $rp->ID, 'regular-posts' ); ?>
<?php else : ?>
<img src="<?php echo get_template_directory_uri(); ?>/img/no-image.png" alt="" />
<?php endif; ?>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>
