Why why would James go after her
He knows what she's been through. He was married.
I know this is a movie, but these things happen in real life too.
Going through a "divorce" and going after a woman who was cheated on and broken hearted.
I get what you are saying. Like maybe James was being rather predatory, going after 'low hanging fruit' of a woman in distress.
But I guess it was their initial chemistry on the tube that attracted him to her, the it just built from there. But also don't forget that there were some pullbacks during the story where James backed off, a couple of times.
Great movie.