I know you shouldn't apply real-world logic to fiction, but the idea that she's being selfish or unfair for not wanting to share HER boyfriend is not a real line of thinking. Unless you're into that sort of dynamic, wanting exclusivity in a relationship is pretty standard. It's honestly kind of wild because she literally just said she'd do anything to be Haruka's number one. But I guess her reserved, non-confrontational nature runs deep enough that she can't just flip a switch on it.
Now if I were Hidaka, I wouldn't go guns blazing into this messy love triangle. That's not really my place. The most I'd do is sit Haruka down for a civil conversation and ask him point-blank where his loyalties lie: Suzuka or Ria. He needs to choose, plain and simple. No need to storm in picking fights over suspected cheating. Hidaka comes across as sensible enough to handle it that way. But then again, the girl he loves is getting played, so emotions might override logic. And let's be real, the author clearly loves drama.
Now let's see how far Haruka and Ria go this time sigh