Is a person considered an infidel if he smiles in response to an inquiry about a mosque for Tarawih prayers, where his relative replied that the imam recites short surahs such as "Qul Huwa Allahu Ahad"?

It is obligatory upon the Muslim to refrain from reviling matters of the religion. Mocking Allah, His verses, or His Messenger is a major disbelief that expels one from the fold of Islam. If your companion intended to mock Surat Al-Ikhlas, or the short surahs, or the Tarawih prayer, then he has committed an act of disbelief that expels him from the fold of Islam. It is obligatory upon both of you to repent to Allah.