No, like parental NTR is literally a super common NTR trope. Parental NTR isn't just about having sex with someone's parent, you actually have to steal their parent in some fashion. It's often the victim's childhood bully doing the NTR which really cements the loss of the relationship. The mother is being intimate with someone in a way that actually betrays their child, because the parent should be defending their kid from the bully instead of being corrupted and seduced by them.
The mission is to seduce Erin's women so that they won't be loyal to him. Set them up to emotionally abandon him so that he'll be compromised. Since the point is to steal the girls it seems that we won't just be having sex with Erin's mom but actually downright stealing her from him, that's parental NTR, or netori in this case.