Koga says during the chapter that he has no idea what happened between Meguro and Sensei. Koga only knows that he was her tutor and first love.
Koga suggests that Meguro should be tutored by Sensei because he believes Sensei is a more suitable teacher for Meguro than him, and is someone, who can help Meguro achieve her goal, and isn't letting feelings of jealousy stop him from doing what he thinks is best for his girlfriend.
I'm not saying that Koga's decision is one that everyone, or anyone, would or should be comfortable with, but it's a little sad to see people not recognizing and praising his decision to do what is best for his girlfriend and having the confidence to trust that his girlfriend won't cheat on him rather than letting feelings of insecurity stop him from doing something that can help her when he notices she's trying to improve herself.