Of course that was wrong but most of her actions were due to pregnancy symptoms, I don't believe she was cheating. Besides, what about the male lead that lied about being eating dinner with his boss when he was just drinking and having dinner with the other girl. That kind of actions bring consequences, she lied about the ring because she was hurt. Wrong, but in the end, she was more wronged than the male lead (however, between the 4 of them, Lee Jang Hyeok was even more wronged imo)
But believe me I started watching and it's totally different. And I don't think he is a chaebol family's long lost grandson. I think that the long lost grandchild is other person but I don't want to spoil it for you! If you watch you'll understand what I mean :)