First you will need to get the custom field for the product ID. You can use the Custom Fields API
to get that info.
Once you have the $drip_post_id, then you could do something like this:
$product_id = $wpdb->get_var("SELECT id FROM ".$wpdb->prefix."dfr_shop_products WHERE id = ".$drip_post_id);
And then $product_id should equal an ID or false.