I don't see what's hot in seeing Mila get ntr'd. As a punishment for her cheating on Paul as a drama sure, but as a kink wtf. I don't get this, just as I don't get what's hot in male protagonist games when protagonist is ntr'ed.
It is true that if your partner knows and is getting pleasure from it is better for sure.
The added point of Mila getting NTR would be from the emotional standpoint, she folllowed her desires, broke up with Paul and in the middle of her sorry life she sees Paul with a new woman, happy.
Realizing what she gave up for some D could break her, but I don't know if this kind of approach is what the author is looking for.
I mean, the NTR Mila is already a shadow of a woman (rightly so), that could be a nice angle, not so much for the porn (at that point she could spiral into even more degenerare stuff if possible) but story wise.