Here is an example for getting the E-mail (First name Last name) in the select user field.
function alter_specific_user_field($result, $user, $field, $post_id) {
$result = $user->user_email;
if( $user->first_name ) {
$result .= ' (' . $user->first_name;
if( $user->last_name ) {
$result .= ' ' . $user->last_name;
}
$result .= ')';
}
return $result;
}
add_filter("acf/fields/user/result/key={field key here}", 'alter_specific_user_field', 10, 4);