Is it just and in accordance with the law of Allah for a husband to leave his first wife for such a long period to sleep with his other wife, while the first wife is in need of him?

If the wife resides in another country for her work, she has no right to the [fair division of nights or resources known as] qasm. If she returns to her husband, he is not obliged to make up for the past nights. Rather, justice must be observed between the wives. It is permissible for a wife to graciously relinquish some of her nights to another wife out of favor. However, if a wife travels with her husband's permission for his need, her right to qasm is not forfeited, and it must be made up. But if her travel is for her own need or without his permission, she has no right to qasm.