Gordon Ramsay had a food critic do the same test to a restaurant he was evaluating for a competition. That's probably where she got the general idea from.
That being said, the secret customer sent by Ramsay wasn't there to poach staff, and although she was indeed very picky and demanding, she was nowhere as unreasonable and as extreme as the pizza customer. The pizza customer literally behaved as she was psychotic by completely lying about what she had originally ordered, not once, but twice. She not only did this at the financial expense of someone else's restaurant but also at the gaslighting expense of the staff member actually taking the orders.
Second, some liars are just pathological. They will lie about anything and everything. Now it's possible she owns an actual restaurant, but it's just as possible that she makes job offers the same way she likes to order pizza.
There is also something wrong with the "customer is always right" way of running a restaurant (thanks to Daniel R Collins for suggesting that great article). I understand the need for good customer service, but some employers just take it way too far and abandon all common sense - often times at the expense of their own employees. And I feel this is what happened in this case, the pizzeria employee knew its employer would side with the customer no matter what, so he behaved accordingly. That's the way he was trained.
So to me at least, the potential new employer is really entitled, selfish, immature, and potentially pathological, but the current employer doesn't sound very good either if he forces his employees to acquiesce to such people in the first place.
In my opinion, the entire US hospitality industry is sociopathic (barring a few rare exceptions) since it cares so little about the employees that work for it (not to mention the larger service industry). And my advice would be to choose an entirely different field to work in and to just keep cooking as a hobby on the side.
That being said, if cooking in a restaurant is his passion, or if he's really enticed or excited by the new opportunity, or if he already knows all the negatives about his industry, and on some level, he most likely already does, I'm not sure I would even say any of those things to him.
Some people really do enjoy the camaraderie of working in a kitchen, the fast pace, the hierarchy, the very long hours, and the obsession of doing a perfect job day in and day out. And if such a person was my friend, I wouldn't want to be the one to rain on his parade, unless he complained about it to me first.
