Is an employee considered sinful if he knows that his private company is defrauding a foreign supplier and illegally taking money from them, and what is he required to do in this situation?

When witnessing reprehensible acts, it is obligatory to advise those who commit them, forbid them from such acts, and remind them of Allah and the Last Day, unless there is a fear of unbearable harm. If they do not respond, their complaint should be raised with the competent authorities, along with evidence, so that necessary action may be taken. Through advising and reporting, one’s duty is discharged, and a great reward is attained. The one who advises may face harm, so he must be patient, for his recompense is good in this world and the Hereafter.