src/Model/Order.php
public function getBillingAddress()
    {
        if (!$this->SeparateBillingAddress && $this->ShippingAddressID === $this->BillingAddressID) {
            return $this->getShippingAddress();
        } else {
            return $this->getAddress('Billing');
        }
    }
public function getBillingAddress()
    {
        if (!$this->SeparateBillingAddress && $this->ShippingAddressID === $this->BillingAddressID) {
            return $this->getShippingAddress();
        } else {
            return $this->getAddress('Billing');
        }
    }
Get billing address, if marked to use seperate address, otherwise use shipping address,
or the member default billing address.