function my_wpf_status_tagging( $order_id ) {

    if ( ! function_exists( 'wp_fusion' ) ) {
        return;
    }

    $order   = wc_get_order( $order_id );
    $user_id = $order->get_user_id();

    wp_fusion()->user->apply_tags( array( 'TAG NAME' ), $user_id );

}

add_action( 'woocommerce_order_status_completed', 'my_wpf_status_tagging' );