I found this on this website (Visajourney)
What happens if we do not get married within the 90 day validity of the K1 visa? A..In this case, you can no longer file for Adjustment of Status based on being a K1.You will have to submit an I-130 relative petition along with the I-485 and all the other paperwork to the local USCIS (INS) office. You may have to have another medical exam, and you will need to be prepared to answer questions about why you did not get married within 90 days. In effect, you will have become an out-of-status alien who is adjusting status based on marriage to a US Citizen.
Some say she has to go back to her home country.
Does anyone know what or how she can stay without being married on a K-1 or is it possible to stay here without being married? Will she get in trouble or have problems and possibly have to go back if she does not get married and stays?