The values of a multiple select list will passed as an Array, you can use the PHP implode() function to convert the Array to a comma separated list.
in the WA_UniversalEmail/WAUE_index_1.php file, change line 94:
$MailBody = $MailBody . ((isset($_POST["Appointment_request_group_Best_times[]"]))?$_POST["Appointment_request_group_Best_times[]"]:"");
to:
$MailBody = $MailBody . ((isset($_POST["Appointment_request_group_Best_times"]))?implode(", ", $_POST["Appointment_request_group_Best_times"]):"");