بسم الله الرحمن الرحيم
It is very bad to pronounce the said sentence, she should do taubah. Oath takes place by the said sentence; thus if she goes against her oath, it shall break and kaffarah shall be mandatory. The kaffarah is to feed 10 poor people full stomach two times or to give wheat equal to the weight of Sadaqah al-Fitr (10633 gram) to 10 poor people or gives clothes to 10 poor (each one shall be given kurta and pyjama or kurta and lungi). And if the woman who took the oath is too poor to pay the kaffarah as mentioned above, she can keep three days fast continuously, thus her kaffarah shall be fulfilled.
ولو ان قال ان فعل كذا فهو يهودى او نصرانى او كافر فيكفر بحيثه اى تلزمه الكفارة اذا حنث الحاقاله بتحريم الحلال . شامى ج 5 ص 492.
Allah knows Best!
Darul Uloom Deoband