While it's undeniable that MC lied to Sungah he didn't do that in order to take advantage of her, he actually managed to help her with her crush. If he's gonna tell her, I think she would feel betrayed and they will fight, but after that they could still be friend in real life. Unless, of course, the reveal coincides with MC saving her from her abusive boyfriend, in which case she will be simply grateful to him.