Keeping silent to avoid an argument

If a mother in law discusses something with her daughter in law and later her son confronts his wife in front of his mother demanding to know what the mother said, the wife remains silent out of respect for the mother in law and also to prevent an argument from taking place between the mother and son. The husband later accuses his wife of being disrespectful for not telling him despite her explaining her reasoning. He still won't see things from her perspective. Please advise if she was disrespectful towards her husband and what should she do if faced with a similar situation in the future?

N.B. The husband never admits to being wrong and cannot be reasoned with.