Obviously Kurokawa is a dick.  If he were a mature adult, he would have talked to Rinko instead of guilt-tripping Arase.  And if Arase were a mature adult, he would have just told Rinko that Kurokawa found out about them and that he feels that they shouldn't see each other any more.  And if Rinko were a mature adult she wouldn't be so close with Arase in the first place.  Honestly, of all the characters in this series I think Hifumi is the only one with her shit together.  And Rinko's sister, her too.