I'd go with a sharpening set. I've never been a professional cook, but the professional kitchens that I've walked through have knives that are often in terrible shape.
In a previous life, I worked in a retail seafood. From what I've seen, most cooking schools teach horrible fish prep and cutting techniques. Fish tweezers are essential for fish prep. I like these that Jon carries:
http://www.japaneseknifeimports.com/other-items/fish-tweezers/fish-tweezers-round-non-slip.html#
I would also recommend a Benriner (large). It's kind of a cheat for prep, but for large amounts of prep, it's invaluable.
I second the recommendations for fish spatulas, microplanes, thermometers; for books, I also second the French Laundry cookbook recommendation. That book really gets you to think about flavors, processes, and techniques.
But of all the things recommended, I do think the best would be to take them to a very good restaurant and sit at the chef's table so they can see how a kitchen works. I'm still fascinated by it today, and it's probably one reason why I love cooking. I also think a real sushi bar, where you can see prep, whole fish broken down, and ask lots of questions of the chef, is also a great learning experience.