/**
 * Prepare form and add my field.
 *
 * @param   JForm  $form  The form to be altered.
 * @param   mixed  $data  The associated data for the form.
 *
 * @return  boolean
 *
 * @since   0.0.1
 */
function onContentPrepareForm($form, $data)
{
    $app    = JFactory::getApplication();
    $menu   = $app->getMenu();
    $pid    = $data->parent_id;
    $parent = $menu->getItem($pid);

    var_dump($data->parent_id);
    // returns the parent_id string of the currently edited menu item

    var_dump($parent);
    // returns null
}