Who is he again? Why is he important here? To make a point that the MC likes Erika? It was done already though. There is no need from this random guy to casually join in your circle. How the heck do you went stay overnight with someone that is not in your circle anyway?
Then it must be Wakasa-sensei since she is an elementary teacher. Then again, such knowledge might be known easily by Rum as the second in command. I mean, Yusaku who is not a teacher know about it too.
What a turn off. The MC knows right from wrong. Yet, he didn't tell it to her even though he knows that she is lacking common sense so much. I guess the MC is just as lacking.