It's an "Exchange", and the sender needs to realize that they need to print extras just in case something gets lost, or even damaged in shipping.
After all, the choices made when packaging and sending the print can have huge effects on the condition of the print. If I recieved a print that had merely been slipped into an envelope and mailed with no protective padding or a stiffener added to keep the print from being folded, and the print was severely damaged, I would not find that acceptable for an exchange.
If it had been well packaged, and it was still damaged, as the sender, I would resend another copy to the recipient. If it never made it, well, I would send one again, with tracking to make sure it did get there.