It is not necessary to have an original proof of purchase for a complaint.
It is sufficient to properly prove to the seller that you have purchased the goods from them and when the purchase was made. This may be provided by an account statement of the transaction, an invoice, a cash receipt, a warranty card, a testimony of a person who attended the purchase, email communication, an email order confirmation and more.
If the seller required an original proof of purchase for the consumer to make a complaint, they would act contrary to the law.