At the moment i found this solution that work for me. I know is not the best solution because i'm gonna to modify 1 core file.
I have modified the "public static function getBasePath()" that is responsable to retrieve sample files path inside the "Sample.php" inside this folder "app/code/core/Mage/Downloadable/Model/"
I have added an IF condition to check the groupID:
public static function getBasePath() { $_groupId = Mage::getSingleton('customer/session')->getCustomerGroupId(); if ($_groupId === '4') { return Mage::getBaseDir('media') . DS . 'downloadable' . DS . 'files' . DS . 'samples'; } }
Any suggestion is really appreciated