You can't use Joomla API outside of Joomla without loading in Joomla. Suggest you explore using CB Auto Actions as it has a Private Message action type for sending PMs. If Joomla and CB are loaded you can use the $_CB_PMS global to send a PM. Example as follows.
Code:
global $_CB_PMS;
$_CB_PMS->sendPMSMSG( TO_USER_ID, FROM_USER_ID, PM_SUBJECT, PM_MESSAGE );