It's kind of disappointing we got exactly zero seconds shown of Rudeus developing it. But I guess it's not the focus of the story, in the end, even if it supposedly is quite a big deal, after his fatal encounter with Orsted.
Cliff didn't seem like a super social person, plus Rudeus might not really want to know too much. He's trying to get over that relationship, after Eris left him behind. The most important thing was that Eris isn't actually attending the same academy. That would be awkward, from Rudeus's point of view, for sure.
You are, kind of, jumping to conclusions here.