In case someone is still looking after this issue…
If you pay attention to the error message on the first post (Fatal error: Class ‘Mage_Invitefriend_Helper_Data’ not found in ...), you can see that the “Mage” namespace shouldn’t have been used there… It should have been Implementek_Invitefriend_Helper_Data instead, which uses the namespace used on that specific module. To tell magento that you have your own helpers for this module, you should add this lines to your config.xml, inside within the config->globals tags:
This way you are telling magento to look for the helpers called inside you module in the Helper folder inside your module. Of course, after this you should create your module’s helper. The path would be: app/code/local/Implementek/Invitefriend/Helper/Data.php, and the content would be:
class Implementek_Invitefriend_Helper_Data extends Mage_Core_Helper_Abstract
And of course, if you are not the guy who posted this thread, remember you have to replace the Company namespace and the module name as appropriate for your module.