Highlight current post ancestor parent menu items

This is probably what you are looking for ..

add_filter( 'wp_nav_menu_objects', 'add_menu_parent_class' );
function add_menu_parent_class( $items ) {

    $parents = array();
    foreach ( $items as $item ) {
        if ( in_array('current-post-ancestor', $item->classes)  ) {
            $parents[] = $item->menu_item_parent;
        }
    }

    foreach ( $items as $item ) {
        if ( in_array( $item->ID, $parents ) ) {
            $item->classes[] = 'current-menu-ancestor'; 
        }
    }

    return $items;    
}

It will add a class to current post ancestor menu item parent. I have tried this in my theme and it is working perfectly.