Thats grim OP.
I know when I have purchased intu vouchers before they have the serial number on the receipt and have told me that if I lost the vouchers they could replace them, and stop the originals, if I quoted that number. Not sure how / where you purchased the Amazon ones, but would be great if you could cancel them, and at least stop her using them!
Worth a note to amazon if you still have any details. You wont have to go into details of the situation.