It's her marriage, she can divorce him because him for whatever reason she pleases. Obviously in the eyes of Allah one should have patience with their spouse so divorcing for petty things or things which can be resolved is not recommended. Ultimately if the thought of her husband having two or more wives is too much to handle then she can divorce him.
In Islam you can divorce your spouse if the marriage is not working or if one is unhappy. Thus if the husband getting a divorce makes the wife unhappy, she can divorce