Is it permissible for an employee to inform the employer—via an anonymous message—about transgressions occurring in the restaurant, such as workers eating from the restaurant meals designated for customers, leaving during working hours, and their continuous use of mobile phones, knowing that he cannot advise the workers, and would he be sinful if he remained silent?

You must forbid the workers from evil and prevent them from transgressions to the best of your ability. If no one else knows about them, or if you are the only one capable of preventing them, then it is not permissible for you to remain silent concerning them, due to the saying of the Prophet (peace be upon him): "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith." If they do not respond to you, then inform the employer, after advising and guiding them, due to the saying of Allah the Almighty: {And cooperate in righteousness and piety}, and due to the saying of the Prophet (peace be upon him): "Religion is naseehah (sincere advice)."