on 19 shimizu says "her" but as far as i remember she didn't specify the sex?
So i guess that's either a typo or the translator implied that the guy didn't hear it? I thought he did.
Japanese has gender-neutral pronouns, English doesn't.
English has they/ them pronouns, which they actually use throughout the conversation, which is why the random "her" is so confusing.
The translator might have thought it just sounded more natural. There's a good chance Kidoguchi knows her sexuality anyway, though. They dated before, and broke up because she isn't into guys. He might even know about the Yuri manga thing, so while she hasn't explicitly come out to him, he may know enough to infer that if she's into someone, it's probably a girl.
Given all that, the translator may have just figured he probably knows, and is cool with it, or just plain wouldn't mind, so if "her" sounds more natural than "they"...
Either way it's probably not worth fretting over too much.