What is the ruling on a friend investing in a plastics factory, such that he profits from a specific type (number 5) as a percentage of its sales, and gets his money back after a year? Is the money earned from this contract lawful? And is the factory owner sinful if the contract was unlawful and he was unaware?

If the contract is a loan with fixed interest-based profits, then it is forbidden usury (riba). Therefore, you must repent and return the principal to the lender. The lender must return to you whatever profit he gained. Your conscience will be cleared for what has passed if you did not intentionally commit the sin.