This isn't really NTR. MC has a bit of an inferiority complex towards his childhood friend Hazuki and feels left behind; he's still playing the same games he played in elementary school while she's experiencing new things. At the same time, his awakening to sex involved seeing Hazuki and his interest in lust in general only grew from there.
MC doesn't like Hazuki and even stopped talking to her entirely after page 9 due to his complex. Interesting book.
Page 29. I'm feeling like the girl had went through an emergence-tier character development (mild version) while she and the boy were apart. She looked so happy when they met again, only to get brushed off and ignore. In the end she just accepted her fate, and we wouldn't know her story since this work is not from her POV.
I also don't think MC really likes her. I even think she and her boyfriend are in love. If the following story exists, I want to see MC can ntr her boyfriend . :)