Person A stated some noble motives behind actions person B found suspicious.
[Person B] answered with very measured words, her smile not quite reaching to her eyes. "If what you claim is true, then you can count me as your friend." The unsaid counterpart of the sentence hung heavily in the air.
I'm not quite sure if counterpart conveys the meaning well. Is there a better phrase to fit that meaning? How would you call the sentence: "If what you claim is a lie, then you can count me as your enemy." in relation to the one given in the quote?