Is the husband considered unjust to his wife by continuing this situation? And what is the ideal solution for dealing with the mother in this case? And is it permissible to temporarily boycott her, or is that forbidden? And how can the mother be stopped when she verbally abuses the wife?

It is forbidden for you to sever ties with your mother, raise your voice at her, or use offensive language. You must treat her kindly and obey her in matters that do not involve disobedience to Allah. Allah and the Prophet, peace and blessings be upon him, have enjoined good treatment of parents, especially the mother. Therefore, you should advise her gently and wisely to fear Allah concerning your wife, and you should be patient and benevolent to win her affection. If the problem is not resolved, then you must separate your wife from your mother as much as possible. It is important to note that some wives may feign victimhood to incite the husband against his parents.