If the main protagonist were the hubby of MC's mom you could call it NTR. But since NTR is only some weird incel double standard it's only cheating.
One problem here with NTR is that it's kind of a vague definition, and just "NTR" doesn't really cover the different kinds of it.
In this specific case, what MC does to Tomoe actually does fall into the broad definition of NTR, just Netori and not Netorare.
With Netori, it's basically of the perspective of the MC being the homewrecker, seducing the already taken LI, and makes her prefer him over time.
We kind of only seem to refer to NTR as Netorare though, which is when the main focus is on the adulterer in the relationship with the homewrecker character being the outsider perspective.
Honestly I think it would have been better of F95zone had Netorare, Netorase and Netori as different optional tags, maybe alongside NTR. But then people might not really understand it that well anymore, when most people have some sense, albeit lacking, of what NTR really is and isn't.