Wait, Shuuna said he didn't know why they broke up, but didn't Hayashida tell him that he ...

he was just lying so yumi did not know that he is the actual boyfriend of kan-chan

I understand what you mean but he said he didn't know exactly in what circumstances they broke up. He knows he was abusive but not the exact details of it ended at least it's what I think he meant.

hmm, perhaps. I don't think he needs details, he told him he abusive towards his previous partner, that's a pretty self explanatory reason behind them breaking up.

I just re-reading this series right now too. He tells him he hit his partner here:

http://www.mangago.zone/read-manga/renai_rubi_no_tadashii_furikata/bt/ebd618fe1b9ef76dd5ee458671769b51_chapter-4/pg-47/

Then he tells admits that he was dating the guy in the picture, yumi

http://www.mangago.zone/read-manga/hadakeru_kaibutsu/mi/sa-c004/pg-7/

Ookay I reread it and here's what i think: he didn't say he didn't know why they broke up but how which changes everything. He's basically saying if there is something you wished you had told him (like apologies) when you guys broke up, I think you might feel revealed to be able to talk to him and truly move on. He's saying even though he wasn't there to see how it ended he still thinks it might do him good to have a chat with his ex.