What is the ruling of Islamic law if one of the partners in a company engages in an unlawful transaction, and how is the unlawfully acquired money considered with respect to the other partners?

None of the partners is permitted to engage in an unlawful transaction. The other partners are obligated to forbid him and advise him. If he does not respond, then they should not allow this transaction to be mixed with their funds; otherwise, they would share in the sin. If they become aware of it after the mixing and are unable to distinguish the unlawful profit, then they must donate an equivalent amount to purify their wealth.