There is room in the world for more than one author.

Many have succeeded at your dream before you. Their success does not limit or diminish your dream. Stopping another person from success does not make you successful. It is simply a waste of your energy, which you could have devoted to writing your book.

You could have ask for his help and insight. You could have collaborated.

Stopping others creates a negative world. It is spiteful, selfish and wasteful.