I work for a company, and the client stipulates that the order must not be manufactured in another factory. However, my employer manufactures half of it in another factory, and I supervise it. He also evades customs without my help. I would like to inquire about the ruling on this work, and whether I should inform the client and customs about it, especially since this is the prevailing custom in factories?

All praise is due to Allah, and may peace and blessings be upon the Messenger of Allah, his family, and his companions. To proceed:

It is not permissible to execute or supervise work that its owner stipulated to be carried out in a specific factory, if the company owner asks you to do so. If your work has no relation to this matter, then there is no harm. However, you must advise the company owner and explain the impermissibility of him breaching the terms of the contract with customers. You can threaten to inform the customers if he does not stop the deception. As for tax evasion, it is permissible if the taxes are unlawful, and impermissible if they are lawful.