That "someone that's not him" was her boyfriend at the time. Erynn fucking him is perfectly normal behavior and it's not even in the same realm as NTR lol. Erynn fucking her boyfriend is neither NTR or cuckolding.
Cucking would be if Erynn were intentionally fucking Ethan in front of MC to emasculate MC.
NTR would be if Ethan used the blackmail to force Erynn to cheat on MC against her will. But Erynn is not dating MC yet, and (more importantly) Ethan hasn't even gotten her to have sex with him yet. "Cheating" and "Stolen love" are kind of critical elements to NTR. What Ethan is doing is simple blackmail... Blackmail that did not succeed now that MC is involved. If there is a later scene where Erynn gives into Ethan and lets her bone her in front of MC in an effort to make Ethan delete the video, then it would be NTR, but that hasn't happened yet (and it's unlikely to).
I'm increasingly certain you have no understanding of any of these tropes. I've honestly never seen someone so stubbornly wrong on these boards
If you want definitions, please feel free to google them.