What is the ruling on working for a company whose manager unlawfully consumes people's wealth, and to what extent is the employee sinful in this, and is he obligated to leave the job?

If you advise someone to abandon a reprehensible act (munkar) and they do not respond, then you should inform the authorities about them, provided you do not fear harm. This is because it falls under the category of enjoining good and forbidding evil (al-amr bil-ma'ruf wa al-nahy 'an al-munkar). By doing so, you would have fulfilled your duty. You are not required to leave your job as long as it is permissible and does not involve deception. You will not be sinful as long as you have exerted your effort in trying to change the reprehensible act.