Hiyama would honestly be a better boyfriend, I remember in the earlier chapters it's nothing but her current boyfriend fetishizing her size a lot as the reason he's with her. Now she cant even have a male friend because he's probably scared of another guy genuinely being nice and giving her attention. Even though Hiyama has some romantic feelings at least he doesn't act on them and respects that she has a boyfriend already. Tagami didn't even look happy when she came back smiling, probably upset he's not the one that made her feel better and he knows if it wasnt her friends it was pribably Hiyama