You're forgetting that she is heart broken herself. When she found out the Yana and Linda had something going on, that hurt her a lot. This is her way of taking some of her angry, pain and heart break out on someone which so happens to be Banri, because she's seeing it like: Banri has a girlfriend but is still suspiciously close to another girl who so happens to be Yana's crush, she's not thinking and thus making up conclusions in her head (like most humans do) that Banri is a lying backstaber but she's actually angry for the most part because she got hurt because she couldn't have Yana, and now she think's Yana is going to get hurt because he couldn't have Linda. So her and the person she loves getting hurt, makes her angry.

And it is hard to tell secrets like that... It's something that will change people's views on him and how they treat him. If your friend started dating a girl you dated and loved long ago, but he didn't know that you dated her. How would you tell me that you've been with his girlfriend and loved her? It's not easy to blurt out secrets...