Responses
If you really want a spoiler...
We are on ch 155 and so-myeong did catch siyoon talking to her mom on the phone way earlier in the story, but we never got to see to what extent he was helping her mom. He did admit to kind of keeping tabs on her but he said it was just because he was worried she was spiraling. So far he does seem to believe her and even tries to help her even though she is not friends with him anymore. So he's not a 100% confirmed traitor but he's already fucked up enough to be trusted again.
Im only on ch25 BUT someone tell me if Siyoon (her best friend guy) is some traitor or smth, i love him but its just suspicious the mom allowed only Siyoon to be close to her since childhood