woocommerce_order_pay_authentication.php

if ( class_exists( 'woocommerce' ) ) {
	if ( is_wc_endpoint_url( 'order-pay' ) ) {
		global $wp;
		$order_id = $wp->query_vars['order-pay'];
		$order_author_id = get_post_field( 'post_author', $order_id );
		$current_user_id = get_current_user_id();
		if ( $order_author_id !== $current_user_id ) {
			// Do something here when user did not place the order.
			// Redirect to homepage etc.
		}
	}
}