I mean, either someone is allowed to use the discount or they aren't. It's black and white. As a TM, I'm not going down because someone wants to save 10 cents on a purchase when they aren't entitled to it.
If you allow one person to use a discount erroneously, why not let everyone? Where is the line drawn then?
She had the physical discount card with her. Now she has it in the app. It's no different.