How can she tell her non-Muslim mother that her husband is going to take a second wife?
Question

                what is the best way to tell my non muslim mother, who is very critical of islam, that my husband is about to take on another wife?
Praise be to Allah.
    There is nothing wrong with you happening to mention to your 
    mother whilst you are talking to her that your husband is thinking of taking 
    a second wife, and telling her that he has valid reasons for doing so, such 
    as not having any children from you, or that he is not satisfied with one 
    wife, and so on. That will be like an introduction for her if she knows. But 
    do not tell her about the matter now, less that open the door to problems 
    for you that you can do without. If she finds out about that in the future, 
    then you can explain to her, one way or another, that Islam is the religion 
    of justice and does not approve of injustice on anyone’s part, and that in a 
    plural marriage there are many great benefits which the countries which 
    claim to be civilized cannot attain. (See question no. 
    12528). Even if she is not convinced at first, she will become 
    convinced as the days and years go by. But you should not express too many 
    objections about this matter in front of her, because by doing so you will 
    provoke her to criticize Islam. Rather you have to show her that you accept 
    this and approve of it, and show her that  it is not affecting your 
    relationship with your husband.