That is the main point of the story though. She wants Sumin to marry her ex husband, and would she marry him if Jiwon just dumped him? No. Sumin wants to have what Jiwon has, so Jiwon has to make the guy and the relationship with him look perfect and desirable for Sumin to try to steal him on her own.
This series is just so enjoyable! And the kidnapping scene....I died laughing. But come on, the episodes have 20 something minutes and they spend 5 min with repeating the scenes from last episode without adding anything and long credits. I wish the episodes were longer and the scenes less disjointed, although it's pretty good for a fluff comedy so I'm not complaining too much. haha