How can a non-Muslim mother, who severely criticizes Islam, be told that her son will marry another woman?

There is no harm in incidentally informing the mother that the husband is considering marrying another woman, mentioning the reasons if any, as a prelude to her accepting the news in the future. When she becomes aware, it can be explained that Islam is a religion of justice and permits polygyny for great benefits. The wife must show acceptance and contentment with polygyny to avoid inciting the mother against the religion.