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If a letter of the message appears in the top line then read the corresponding letter from the bottom line and vice versa. She made a mistake with the last letter however and forgot to code that. It's just a simple reverse alphabet code.
There's a small cafe near where I work and the German owner speaks in a similar code. The other day, I asked him what he had available for breakfast and he replied, "vfmnx". I think I'll have to borrow an Enigma machine....I don't suppose?