Not sure how this would differ with multi-site, but this is how you’d do this outside the loop normally:
<?php
# get post data
$temp_post = get_post($post_id);
# grab the author meta
$user_id = $temp_post->post_author;
# grab the field you're looking for
$first_name = get_the_author_meta('first_name',$user_id);
# display field
echo $first_name;
?>