Fix my Prestashop Store payment gateway

My previous developer created the module and it is able to pass the cart details correctly to the payment gateway, however when the payment is processed by the gateway the information does not get passed back to my Prestashop back office and update the orderstaus,. For example I will get an email confirmation from the bank that the payment had gone through but I the cart status is not updated in the back office. I have a prestahop technical support package and they told me that the developer needs to look at the code in the validation.php. Basically I need the gateway to update my back office when a transaction in complete.