You can say the same to Don as well. He's cheating on Nami and doesn't even tell her about it. Not to mention that he doesn't even love her and will never be able to love her but don't tell her that either.
Don is a jerk for cheating on Nami, sure. For the rest I think she was aware of his feelings from the start but still entertained hope. A relationship is not based on just love so I don't think neither of them was being insincere. She was a caring friend when he needed and she was in love. She just ceased the moment to switch the relationship to what she expected. Don didn't object probably by gratitude and because she is the next best thing after Haero. It's sad that she was stuck there for 10 whole years in a relationship that was holding up only by her hard work, hoping that Haero won't ever come back...
That is why it is Haero's responsibility not to lead her on and tell her straight that she has no chance. He didn't say anything to her and just took advantage of kindness for 10 years even after coming to realize that he will never love her. Had he made it clear and she still stick with him, it will be his fault. But she's the only one who made clear her intention while he remain silent.
"...it will be *her fault."
Yeah. Well we'll see what she says to Haero in the next chapter but I have a feeling she's going to ask him to back off and leave them be
That depends on the author. But if the author were true to her character, she'd learn to back off on her own once she confirms Don's feelings. That's because she promise Don she would. And she knows Don love Haero very deeply.
Haero is such an annoying selfish guy