If you are using Magento's default customer Account Information tab to see this information in admin then you can see this file:
/app/code/core/Mage/Adminhtml/Block/Customer/Edit/Tab/Account.php
In this file you can see these 2 lines which set the form data in admin:
$form->setValues($customer->getData());
$this->setForm($form);
So you can add below code above these lines to update any customer attribute value to be seen in admin Account Information tab:
$admin_user_session = Mage::getSingleton('admin/session');
$adminuserId = $admin_user_session->getUser()->getUserId();
if($adminuserId ==68){
//like below code you can set particular customer attribute value as per your requirement
$customer->setFirstname('test');
}
Note: This code will only manipulate the customer data before showing on Account information tab it will not update the actual customer data, if you want to update use
$customer->save();
after the above code
Also this is the core file, so please override in your module as not recommended to modify core files.