the "yet" is the important part. if they have the potential to be a love interest at all (and let's be real, she is 100% a love interest from the start of the story.), then having them fucking other characters counts as ntr, outside of them having a pre-established relationship at the start of the story (like a married woman, or with a boyfriend, before the story starts.).
the fact that he becomes a boyfriend partway into the story, and she's definitely a love interest from the start, makes it netorare.