If you can modify the code where the WC form is displayed you can use acf_form() with the form option set to false. You will also not be able to use the WC hook and you’ll need to create an acf/pre_save_post action https://www.advancedcustomfields.com/resources/acf-pre_save_post/ to do the work otherwise ACF will go ahead and save the values to the order no matter what you do in the WC filter, and all you’ll really need to do in the filter (i think, don’t hold me to this) is to return the ID for the user like.
// this is just a guess
$post_id = 'user_'.$user_id;
return $post_id;
