She stipulated the condition that if he married a second wife, that second wife would be divorced
Question

                A man got married and they stipulated the condition in the marriage contract that every woman he married would be divorced, then he got married (to another wife). What is the ruling according to the four schools?
Praise be to Allah. 
          Shaykh al-Islam Ibn Taymiyah was asked 
          this question and he replied as follows: 
 
          This condition is not binding according 
          to the madhhab of Imaam al-Shaafa’i, and it is binding on him according 
          to the madhhab of Abu Haneefah, so that when he gets married the divorce 
          takes place, and when he takes a concubine she becomes free. This is 
          also the view of Maalik. With regard to the madhhab of Ahmad, no divorce 
          or manumission takes place, but if he gets married or takes a concubine, 
          the matter is in the woman’s hands: if she wishes, she may stay with 
          him, or if she wishes, she may leave him, because the Prophet 
          
          (peace and blessings of Allaah be upon him) said: “The conditions which 
          are most deserving of fulfillment are those by means of which intimacy 
          becomes permissible to you.” And because a man got married subject to 
          the condition that he would not take another wife; he referred the matter 
          to ‘Umar, and he said, “Rights and duties are to be defined in the light 
          of conditions.” 
 
          So there are three views concerning 
          this matter: 
 
          That divorce and manumission take place.
 
          That they do not take place, and that 
          the woman does not have the right to leave him.
 
          The most just of the three views, which 
          is that divorce and manumission do not take place, but that the woman 
          has the right to insist on the condition: if she wishes she may stay 
          with him or if she wishes she may leave him. This is the most moderate 
          opinion.