so concern

What is the name of your state? MT
My cousin is on a Fiancee Visa but if her marriage will not work, can she still stay here in the US?

No, absolutely not. If she is not married to the petitioner within 90 days or if she gets married but then gets divorced, she is not eligible to change status or adjust status on any other basis without first leaving the US. The fiance visa only permits its holder to enter the US for the express purpose of marrying the petitioning US citizen fiance and to do so within 90 days from the date of entry.

Please let me know if you have any additional questions or if I may be of further assistance to you in this matter.