Re: In her case, there's a problem..
Obviously her case, although it looks exactly the same, is de facto different.
Assuming both the phones and the cases are new I would return the cases to where I got them a.s.a.p., with both phones and cases present, to, if necessary, empirically prove my point on the spot.
If I were on the other side. If I had sold those cases to the customer I would never let it get that far. It's not worth it. Instead I would see an opportunity. An opportunity to make the best of a bad situation. And turn it around 180 degrees: no losers, only winners out of this li'l problem. How? By not being objectionable to the complaining customer in the first place of course. But by showing you understand their problem and then simply taking back those 2 cases, one of which is quite obviously faulty (keep 'm separate, but together! Mark the faulty one), and let the customer select another pair of matching cases in the price category directly up from their old cases and go home happy. No paperwork, nothing. Not more than 10 minutes.
The customer would go home happy with his, their, pretty new expensive matching cases while being amazed about how easy-going and quick problem-solving I, the sales guy, had been. Because that's rare. So they would tell their friends. And that's the best kind of 'advertising' you can get! All for the price of 2 minutes worth of paperwork.
Saves both parties a lot of unproductive time arguing too.
But if I were that case and reminded all day everyday of how I look by my mirror image being around I would freak out too.