Oh please I'm angry now. How can be Siwon so stupid?

Exactly! Why wouldn’t he get it right away and trust in Ka jinha. Also what I don’t understand, why didn’t Ka Jinha tell siwon right i there that that was the douche? That way they could protect themselves better. I mean i know the writer wants to add drama but this is stupid drama though.

Communication issues

I think that if Jinha told him, Siwon will be angry. He'll get angry cause he didn't tell him earlier and it would be too much drama anyways.