Is a woman's marriage to a married man without the knowledge of his wife, and his failure to treat both wives justly, unlawful or lawful?

There is no objection to a man marrying without the knowledge of his first wife, but he must be just between his wives, otherwise, he commits a sin. The warning for one who is not just between his wives is that he will come on the Day of Resurrection with one side of his body drooping. Therefore, you must remind him of this warning so that he may fear Allah and be just between you both.