Send Cancellation email notifications

Was using the following code as found in the forums:

add_action( 'app-appointments-appointment_cancelled', 'app_custom_send_email_admin_cancellation' );
function app_custom_send_email_admin_cancellation( $app_id ){
global $appointments;
global $wpdb;

$sql = "SELECT * from " . $wpdb->prefix . "app_appointments where ID = '".$app_id."'";
$app = $wpdb->get_row( $sql, OBJECT );

$date_time = mysql2date($appointments->datetime_format, $app->start);
$client_name = $appointments->get_client_name( $app_id );
$provider_name = $appointments->get_worker_name( $app->worker );
$headers = array('Content-Type: text/html; charset=UTF-8');

$multiple_recipients = array(
$appointments->get_admin_email(),
'scheduling@example.com',
'officemanager@example.com'
);
wp_mail(
$multiple_recipients,
'An Appointment was cancelled',
'The following appointment was cancelled by the user. <br /><br />
Name: '.$client_name.'<br />
Date/Time: '.$date_time.'<br />
Location: '.$provider_name,
$headers
);
}

But after the plugin update, it's throwing a 500 internal error:

[07-Oct-2017 04:02:21 UTC] PHP Fatal error: Call to undefined method Appointments::get_worker_name() in /home/xxxx/public_html/wp-content/mu-plugins/myCustomCode.php on line 110