What is the ruling on the second wife preventing her husband from spending the night with his other wife on her designated night, and what should she do given the situation as described?

A husband must be just between his two wives concerning their nights, so each wife has a night. If the wife whose turn it is refuses her husband to spend the night with her, her right to that night is forfeited due to her disobedience (nushuz), and the husband may spend the night with the other wife. It is not permissible for the husband to spend the night with your co-wife on your night unless there is a necessity; otherwise, he sins. As for visiting her during the day, it is permissible out of need, but he is not allowed to have intercourse or prolong the stay. You have the right to insist on your right and prevent him from going to her on your night, or to allow him to do so. The advice is not to be a cause for increasing problems for your husband, and be gentle with him.