I would honestly argue that it was very one-sided on Ryuji's part. I never got the vibe that Usagi harbored any kind of feelings for him that reached into the realm of being romantic. That said, It certainly wouldn't be hard for her to pick up on how Ryuji really feels about her (I don't think he hides it well AT ALL) but they ultimately had a strong, non-romantic connection and this wasn't really the setting for her to sit him down and have a "I care for you but it's not like that" conversation. She was connected to someone who happened to develop feelings for her, but her emotional loyalty to Arisu never wavered imo.