Hi,
Check the code already in order / show_user.php
You can see this line:
<td><span id="hikashop_order_customer_name"><?php echo @$this->order->customer->name; ?></span></td>
or this one:
<td><span id="hikashop_order_customer_email"><?php echo @$this->order->customer->user_email; ?></span></td>
So you can see that the user data is in $this->order->customer
So you can do this:
<tr class="hikashop_order_customer_melli">
<td class="key"><label><?php echo JText::_('USER_CODE_MELLI');?></label></td>
<td><span id="hikashop_order_customer_melli"><?php echo @$this->order->customer->user_code_melli; ?></span></td>
</tr>