Thank you for your posting, it is always good to hear both sides of a story.
I am now going to respond to your question of what more can I do?
a) Get the vet records from the purchaser either faxed or scanned into an email for you
b) Receive permission from the owner to talk with her vet personally
c) A replacement puppy
is not much of a health warranty, if that is their only option. You and I know very well that puppy owners bond with their selection very early on; therefor the likelihood of a return of a puppy is slim to none. You have heard from at least two breeders here, and now myself that our guarantees work differently. I by contract will refund up to the purchase price of the dog for serious and debilitating diseases, and depending upon the situation-ie the disease was lethal and the pup died, will also offer a free future puppy.
d) If the contract and health guarantee was not sent out well in advance of the puppy visit, make that happen. Also spend some time with would be serious purchasers going over what you will cover and how, and what some of the clauses mean in your contract. A contract is at the minimal end of the scale a document that outlines what is expected from purchaser and seller. What happens if? And yes not every situation can be covered,but lawyers can help with the wording so that your intent is clear.
I do hope that you and your purchaser are able to come to a satisfactory resolution for this issue.