The message didn't lie.
WITH cte AS (
SELECT cident, "month"
FROM orders_extended io
WHERE io.ident = 1 -- 1 will be replaced with a function parameter
)
SELECT o.*
FROM orders_extended o
INNER JOIN cte ON (o.cident = cte.cident and o."month" = cte."month")
ORDER BY o."month" DESC, o.cname