So this is assuming you’re using the Relationship field type with the return format set to Post Object:
<ul class="medium-block-grid-5 blog-posts">
<?php if ( $products = get_field('related_products') ): ?>
<?php foreach( $products as $p ): ?>
<li class="post">
<a href="<?php the_permalink( $p->ID ); ?>" title="<?php the_title_attribute( array( 'post' => $p->ID ) ); ?>" ><span class="featured-title"><h3><?php echo get_the_title( $p->ID ); ?></h3></span></a>
<?php if ( has_post_thumbnail( $p->ID ) ) : ?>
<?php echo get_post_thumbnail( $p->ID, 'regular-posts' ); ?>
<?php else : ?>
<img src="<?php echo get_template_directory_uri(); ?>/img/no-image.png" alt="" />
<?php endif; ?>
</li>
<?php endforeach; ?>
<?php endif; ?>
</ul>
Summary is that you don’t need to put this inside a WordPress loop, you can just iterate over the related posts and call the desired functions on that specific post (in this cast product) ID.
