Deleting all CPT related to user deleting

Using the action hook delete_user is the right way to do it, but there are a couple of things missing. The main one being that you haven’t set up any $args for the WP_Query that fetches the custom post type entries with the user-id metakey of the deleted user.

add_action( 'delete_user', 'delete_his_posts');

function delete_his_posts($user_id) {
    $args = array(
        'post_type' => 'your_custom_post_type',  // Replace with the name of your custom post type
        'meta_query' => array(
            array(
                'key'     => 'user-id',  // Your metakey
                'value'   => $user_id,
                'compare' => '='
            )
        ),
        'posts_per_page' => -1
    );

    $the_query = new WP_Query($args);
    
    if ($the_query->have_posts()) {
        while ($the_query->have_posts()) {
            $the_query->the_post();
            wp_delete_post(get_the_ID(), false);  // Soft delete. If you want hard delete, change false to true
        }
        wp_reset_postdata();  // Important! Resets the post data after our custom query
    }
}