function seifsa_complete_order( $user_id, $tags ) {

    if( in_array( 'Tag Name', $tags ) ) {

        $orders = get_posts( array(
            'numberposts' => 1,
            'meta_key'    => '_customer_user',
            'meta_value'  => $user_id,
            'post_type'   => wc_get_order_types(),
            'post_status' => array_keys( wc_get_order_statuses() ),
            'fields'      => 'ids'
        ) );

        if( ! empty( $orders ) ) {
            $order = new WC_Order( $orders[0] );
            $order->set_status( 'completed' );

            wp_fusion()->user->remove_tags( array( 'Tag Name' ), $user_id );

        }

    }

}


add_action( 'wpf_tags_modified', 'seifsa_complete_order' );