Hi lemmersma,
I got into the same exact problem.
In case you have not already resolved it, this is how I approached it:
function get_user_s2m_field($fieldName,$userID = null)
{
$userID = $userID == null ? get_current_user_id() : $userID;
$custFields = get_user_meta($userID, 'wp_s2member_custom_fields', true);
return array_key_exists($fieldName, $custFields) ? $custFields[$fieldName] : null;
}
This way it works in any sub-site of your network.
Next time I would suggest you to post your question on the s2member official forum: you will have many more chances of getting a proper and fast feedback.
I hope I was helpful.