Is the decline in customers and financial losses in a business after hiring a worker related to religion?

The actions of a specific person have no bearing on the increase or decrease of profit. Believing so is a form of forbidden pessimism, as the Prophet, peace and blessings be upon him, said: "Superstitious omens are polytheism (shirk)." Furthermore, the presence of sins in a place removes its blessings, for the Prophet, peace and blessings be upon him, said: "Indeed, a man is deprived of provision because of a sin he commits." Your brother should be patient and manage his trade through ordinary means, for provisions flow according to Allah's decree.