I wonder if maybe that one customer is just having computer problems?
Our system would not issue a download link until after PayPal confirmed their payment was completed. If there is no payment for that person in your PayPal account history, and if you did not send them a link manually from your Seller Admin > "Send free download link", that indicates whatever link they received was not issued by our system.
Perhaps if you could ask her to forward the "autoresponder" message she received with the download link she used, then you could forward that to us if you still need help figuring out how she got that link.