Done! Thanks to the ACF support for this.
This is the way:
<?php
$user = get_field(‘YOUR_FIELD’);
echo $user[‘user_nicename’]; // displays nicename of user
?>
The user field returns an array which contains the following information:
+ KEY ++ DESCRIPTION +
ID User ID
user_firstname First Name
user_lastname Last Name
nickname Nick name
user_nicename Nice name
display_name .Display Name
user_email Email Address
user_url Web Address
user_registered Date Registered
user_description Description
