Yeah, Sungji x Seju won't happen, but Sungji might be the person who will help both of them not only move on but also heal their friendship (without benefits). If this woman in black is someone who will do something to Sumin, I'm almost sure Sungji and Seju will work together against her and the entire Baek family. Sungji may seem to be such innocent girl, but I have a feeling it's unwise to mess up with her.
I also think Seju might want to use Sungji to get Sumin back at least at the begining. She first thought she'll get Sumin back no matter what, but after Sungji found them in the same bed, she told that Sumin in the elevator. Seju's game is not over yet. She's just waiting for a right moment. But Sungji now knows how manipulative she is. She's probably expecting her to do some moves so she won't be surprised ever again. I liked how she stood up to her and told her right in the face she pity her. With Sumin back, Sungji is most likely a wall between them and what do you do with walls when they stood in your way? You crush them. So Seju will try, but won't succeed.