Honest opinion, I still think Jinah is slightly more at fault. Since relationships are bound with the help of trust (Although, It's understandable due to her past relationship). She should've considered the rumors because she knows that actors and actresses experience having fake rumors all the time. It's normalized. That aside, Seung Hyun probably still should've told her, just to ease her mind about the situation.
Additionally,
to the girl who likes Seung Hyun (the short haired woman) I don't know if she's trying to pi** Jinah off by flirting with our ML or she's just completely dumb-