Was Doumyou a necessary character in this manga?
I mean, she served a function in the plot. "Necessary" is subjective, but she does illustrate how an outside perspective can be blind to the kind of abuse that goes on inside a relationship.
From her perspective, Midori is a smooth talker who swiped the guy she said she liked, possibly cheated on him with another woman. (and then skipped out on work to flounce around with her girlfriend, while not telling her fiancee where she is or when she's coming back.)
From our perspective, we know Midori was already dating Tazune (and having second thoughts) when Doumyou told her she liked him, and we know that after their relationship was "locked in" Tazune turned into an emotionally (and later physically) abusive shithead who drove Midori away, and that Maki is a sweet person who values Midori and cares for her in a way Tazune clearly doesn't.
Not saying Doumyou is a saint, mind you, she's definitely done some questionable things, but she also definitely has no reason to have a positive opinion of Midori at this point. She doesn't really have any way to know what an abusive dick Tazune is in private, either.
And this is pretty typical in a lot of abusive relationships. The abuser twists things around to hurt the reputation of the abused.