No gift voucher customer... WIT?!?

Customer with problem left, 5mins later her daughter in law phoned and had a big argument with me. Customer have lost her gift voucher and still want me to serve her. We said we can't do it without paying if she doesn't have the gift voucher with her. So her daughter in law was saying she definitely came on 19th April to buy a £20 gift voucher. But but?!? That's not even proof right? Just cause she said it? Can't prove it was definitely 19th April, and it was definitely £20. And we check records only a £5 gift voucher was bought on 19th April... T.T Was busy so I said I'll tell my manager to phone her back. So what the hell are we suppose to say to her? She either wants service or money back.