So, thank you for all the very useful answers.
The young man in question, who wants to write music, has already studied complementary harmony using textbooks that I have seen, in particular the harmony manual by Ettore Pozzoli and that of Francesco Tissoni.
Personally, I find Rimsky-Korsakov's manual totally exhaustive for choral writing and the chromatic harmony of late Romanticism. Furthermore, I would advise against Schoenberg for studying harmony from scratch, but in this case, I consider him fundamental because he is the only treatise in the world that explains in practice how a musical modulation is realized. I do not think they should be snubbed because they are "old," given that gentlemen like Webern, Berio, and Boulez studied these books and learned something.
That being said, I find Schoenberg's Structural Functions of Harmony to be a true Bible of nineteenth-century harmony, much more exhaustive than De La Motte, who offers excellent insights but is written by a theorist and not by a real composer, so he "fails" from a practical point of view.
I also believe that claiming Schoenberg's manuals are useless today is like calling Schoenberg a poor teacher, when instead we well know that he is the exact opposite, namely one of the greatest teachers of musical composition in History. I prefer to entrust a young student to "his hands" rather than to those of any De Ninno or Tissoni.
Naturally, Schoenberg explains harmony in a complex way, so if this young man who asked me for advice has talent and good intuition, he will be able to follow the lessons proposed by Schoenberg in these books; otherwise, he will have to leave composition and do something else entirely (Schoenberg himself rejected students who requested simpler, more scholastic lessons). The same goes for Rimsky-Korsakov; we are not talking about some old trombone player but the Master of two figures like Stravinsky and Prokofiev.
Personally, I prefer to steer young students toward the path of great composers and masters rather than the path of the great academic unionists, as I call them, authors of cheap little treatises that teach how to harmonize figured bass with anti-musical solutions completely detached from musical reality.
Those who criticize Schoenberg's manual tooth and nail (and there are many who do) have evidently not managed to understand it deeply or use it in a useful way.
I repeat, those who consider Schoenberg's manual too "advanced," difficult, complex, etc., do nothing more than doubt the pedagogical value of Schoenberg himself, given that he used that book with his students.