This is an exercise that popped in my head because:

1. Something similar happened with me. When I was about 19, I had a Nationalist mentor of sorts online who very articulately explained his views. Even though I wasn't totally consistent over the years, it pretty much stuck. Many of his sources weren't obvious.
2. I think it's good to remember that some of our ideas are pretty inherent in Western history and cannot be easily banned.

One that sticks out to me is The Republic. If you said, "we acknowledge the strengths of many of the ideas in the Republic that Western civilization foolishly abandoned in the last 100 years" - you would already be saying so much about what needs to be corrected. Plato's major dialogues as a whole have so much that I think were intentionally removed from education to make a dumber, controllable mass. Of course, Ayn Rand, the System's false alternative of choice to Leftism, misrepresents them as the problem.

From there, you could work through any number of books from Mortimer Adler's Great Books of Western Civilization to show what this vision looks like in practice. Plutarch and the Stoics would be excellent choices - with the understanding that the Stoics are describing an inner attitude but that fulfilling your duty requires the wish for great actions, like the ones Plutarch attributes to men like Lycurgus.

Having handed our young friend these books, I would give them Schopenhauer's Counsels and Maxims and Gracian's The Art of Worldly Wisdom to explain how to survive when you are not part of the democratic mass herd. Ideally, I would later give them Schopenhauer's other writings but with the caveat that they should not buy into his despair because beyond the negation of the animal will is the possibility of another will - the will to power, to excellence, to Higher Civilization, that Nietzsche describes and hopefully they will one day also read about.

If they learned well, I would also give them Max Stirner, with the caveat that he was trying to undo the proto-Leftism of his time and his egoism should be taken with a grain of salt. Hopefully, like so many great historical figures, they will absorb this and be creative enough to build their own Ideal after deconstructing Leftism and not fall to nihilism.

It would be also wise to have them read books on practical topics, such as sales and marketing, to understand how to persuade and counter dubiously motivated persuasion.