Are there any other options available for him within the 90 days after his fiancé visa?

Your friend has to go back home. A K-1 fiance visa is very restrictive: it cannot be extended, and your friend cannot change status in the U.S. to a student visa or any other visa. He could not even adjust status (receive a green card) on his parent's petition or if he married some other U.S. citizen than his present fiancee: if his immigration visa were ready for him, he would have to leave the U.S. and receive the visa in the U.S. Embassy in his country. If his parents file an immigrant petition for him tomorrow, he would not receive an immigrant visa for about 8 years (much longer if he is from Mexico or the Philippines). He cannot legally stay in the U.S. while waiting for his immigrant visa; if he stays, he will become ineligible for a green card. So, it is likely the best for your friend to go back to his country and to try returning to the U.S. as a visitor, or a student, or in one of the many other visa categories depending on his age, education, financial status, and professional background.