She doesn't have the right because she incredibly hypocritical. It's fine for her to date a married man but it's not right for him to date the same married man. It's fine if she is the other woman and the wife should just accept it but it's not okay that there is a third other person.
The wife nailed Mari's personality right on the head. She thinks she's the only victim and tries to deflect blame onto others instead of taking responsibility for her actions. Junyoung at least takes responsibility for his actions and feels guilt for his perceived wrongdoings.
Honestly she doesn't have a right to feel betrayed she literally said that just because you marry someone doesn't mean you're shackled to them, guess what dating is even less commitment!!! If she thinks a wife has no right to tell her husband not to sleep with others then she has even less of a right hahaha but of course she doesn't see it like that because she's very immature
I am not defending her actions when it come to the affair. Yes, she was wrong for dating man even when she knew he was married. But that doesn't mean she doesn't have a right to feel betrayed by her friend. He thought he was dating a man that his best friend was dating an in love with which is fuck up in of itself. No matter who she is as a person what he did to his friend was fucked up. It doesn't matter if if feels bad about it. Things doesn't stop being what it is simply because you feel bad about.