This is 100% NTR. The romance between Shishido and Rindou cuckolds Sohei, the boyfriend character (who seems to be an insecure, possessive creep).
NTR is short for netori, which means "sleep stealing" and includes all forms of cheating/cuckolding/adultery. So this is definitely an NTR scenario.
It isn't netorare, though. Netorare is the "bad" kind of NTR. Lots of people hate netorare because it takes the cuckold's point-of-view, often encouraging reader identification with someone experiencing the worst kind of betrayal and heartbreak.
People often claim to hate NTR, but those who do are almost always wrong. They really just hate edgy, feel-bad netorare stories. And again, this doesn't appear to be one of those.