A friend made an app based on some calculation method he found in a youtube channel. Shame on him for not crediting the author, but months later this influencer

A friend made an app based on some calculation method he found in a youtube channel. Shame on him for not crediting the author, but months later this influencer created an app of his method and wrote a menacing review for my friend's app, backed with thousands of upvotes. He found other apps implementing the method received similar threats as top reviews.