I couldn't let her do that and told her that that coupons typically are a one-shot deal; the expiry date just means that you missed your chance to use it. But I didn't tell her that my order came out to less than I thought it was going to be.
I was supposed to get a free drink and fries when I ordered a sandwich. At my table, I checked my receipt and sure enough, I got a free sandwich when I ordered a drink and fries!
I wonder if she's on a mission to subvert Mickey D's? :-)