Disable order custom field

In our website we have a promotion like "recruit-a-friend".

The first time a new user buys something may indicate the email of a friend during the checkout process.
In this way the friend obtains a discount of 10 Euro for each new users indicate his name..

I added a custom field in order to manage this information.

I would like to show this field only during the first purchase (after the first purchase the promotion is no more valid)

Do you have any suggestion?

I thought to use the "access level" but I need to use in the opposite way (if a user is inside the group "already purchased" he cannot see anymore the custom field)

Alternatively I also thought to modify the view, and use the function:

$user =& JFactory::getUser();
$userGroups = JAccess::getGroupsByUser($user->id, true);
if (!in_array(9,$userGroups)) {

in this case I don't know how to disable only the specific custom fields (I have 2 custom fields).

Thank You

thank you!

I solved the problem, i used a the group ti check if a user has already bought something (i use akeeba subs, and the first purchase in my site has to be a subscription, so I can add the user to a specific group and then test it)

The code is a little bit different, I hope it may help someone else.

<?php
$type = $this->type;
foreach($this->extraFields[$type] as $fieldName => $oneExtraField) {

// Stefano Start
$user =& JFactory::getUser();
$userGroups = JAccess::getGroupsByUser($user->id, true);
if ( !(in_array(11,$userGroups) and ($oneExtraField->field_namekey=='order_portaunamico')) ) {
// Stefano End

?>
<tr class="hikashop_checkout_<?php echo $fieldName;?>_line" id="hikashop_<?php echo $type.'_'.$oneExtraField->field_namekey; ?>">
<td class="key">
<?php echo $this->fieldsClass->getFieldName($oneExtraField);?>
</td>
<td>
<?php $onWhat='onchange'; if($oneExtraField->field_type=='radio') $onWhat='onclick';
echo $this->fieldsClass->display($oneExtraField,$this->$type->$fieldName,'data',false,' '.$onWhat.'="hikashopToggleFields(this.value,\''.$fieldName.'\',\''.$type.'\',0);"');
?>
</td>
</tr>
<?php

// Stefano Start
}
// Stefano End

} ?>