What specific event caused the narrator to doubt the power of Gurus in the face of destiny?

As described in Chapter 2, the narrator's faith was shaken by a specific tragedy that happened to a close friend. This friend, who was the son of a Guru, lost his own healthy and virtuous son to a sudden fever. Despite trying all human remedies and even having the Guru sit by the son's side, the child passed away. This event made the narrator question the benefit of a Guru's company, wondering what a Guru could possibly do against the force of past karma and destiny, which he saw as an irresistible system governing all beings.