with a stipulation giving her the right to divorce if her husband takes a second wife, then it is essentially requiring him to get her permission before taking another wife, on pain of dissolving the contract and granting her a divorce. If she has not stipulated this in the contract, then she has waived the requirement of her permission.
The problem is whether she is fully informed about her rights. Nowadays, she most likely is not, since the Muslim world is in a basic state of intellectual collapse. Wise qadis in the past should and probably would have taken her lack of information into consideration when judging her request for a divorce in such a case. But these days, we don't think. All we do now is "copy and paste".