I think you'd be surprised at international rates, they're really not that much anymore (with my Comcast plan it would only be $0.08 per minute to call the UK, guessing reverse is similar, just wouldn't be with Comcast). Perhaps she could make a voice call through Skype too. Bottom line, the call will be WAY cheaper than losing out on what she spent.
Perhaps I'm dreaming here, but is it possible the store reversed all the charges when they didn't make the flight?
US-U.K calling rates don't tell us anything about U.K.-U.S. calling rates. Thhe two are completely unrelated.