Remember, though, that Doumyou told Midori she was interested in Tazune before she knew they were dating, and the next thing she knew, Midori was pregnant and marrying him. I don't think we explicitly were told the timing of everything, but whatever the specifics are, Doumyou has two possible conclusions: Midori either went after Tazune after doumyou told her she was into him, or (what actually happened) they were already dating and Midori didn't say anything when she mentioned her interest. Either one of those is kinda shitty and might stick in Doumyou's craw
Her next role in the story was when she confessed her feelings. She says she wants to confess to get over her feelings, and that she'll give up after. She explicitly asked Midori's permission to do so, and she was like "eh, sure, why not haha"
Then she sees the kiss, and she -still- doesn't contact him about it, despite having his contact info, but then he comes into the workplace putting out feelers for where his fiancee is, asking questions he really should have the answers to, and honestly, from Doumyou's position, this has gotta be looking pretty bad. I think there's definitely some self-interest in her actions, but she also definitely has good reasons to think Midori is kind of a shitty person that's mistreating him. (although the reality is the other way around, but that's the thing about assholes like him.)