When you boil any food, many of the food's nutrients, vitamins, minerals will go into the water.
When you steam, most of the nutrients stay in the food.
Same for stir-frying if done fast and lightly.
Roasting destroys a lot of the nutrients. It is not evil. Just do it once in a while.
So if he had to put in order from the best to the less desirable cooking method, I would put it this way:
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I know this is technical, but let's go technical here.