P.S. Yikes, I just realized that unless your dad wrote down the whole number someplace (and why would anyone ever do that?) or like they said, stored it online somewhere with another retailer, there's no way to prove which card was his.
All your hopes are with the data transmission or storage of that last transaction he did. And who knows what kind of mischief the station clerk could do to mess that up.
Even with the receipt? So you think it's possible that she undid the sale? If so, hopefully there will be a transaction where it was reversed.