What is the ruling on benefiting from a person who works in an unlawful profession and all his money is unlawful, such as him being the cause for obtaining a lawful job, and are there similar examples in the books of Fiqh?

If all earnings are from unlawful sources, then it is not permissible to transact with the owner regarding his wealth. However, dealing with him and benefiting from him in matters other than his wealth, such as seeking employment through his intercession or benefiting from his knowledge and expertise, is fundamentally permissible as long as it does not involve compromising one's religion or falling into sin. The Prophet, peace and blessings be upon him, benefited from the care and protection of his uncle Abu Talib, and he entered Mecca under the protection of Mut'im ibn 'Adi, and Abu Bakr entered Mecca under the protection of Ibn al-Dughunnah, and all of these individuals were polytheists.