Yu-ji being dead was quite unexpected. She died what... a year after high school or something? It'll be interesting to see how Seol-a deals with that bit o news.
I got the distinct feeling that Yu-ji was being very sarcastic when she said what she said. Sort of like, there was a role in the conversation she was overhearing that no one else was filling and she decided to step in and show them that it could be a two way street. Not only were they "using" Seol-a but Seol-a was using them for friendship so their entire group is fake and based on nothing. I don't think she believes that Seol-a was using them or that she herself, on a personal level, was using Seol-a. She was, in my opinion, voicing how disgusting and harsh that that possibility or truth, sounds and looks. If we are lucky, maybe that old acquaintance will elaborate for us on Yu-ji's personal feelings about the entire thing because you don't make a surprised face like that when caught and meaning what you said. Yu-ji's face was shock and probably realized in that instance that her and Seol-a were never going to be friends again.