We only find that Lesley May has gone over to the "dark side" at the end of Broken Homes, when she assaults Peter Grant as he's trying to apprehend the supervillain, the Faceless Man, allowing the latter to escape. It was quite a fall.
The ostensible reason was that Lesley thought that the Faceless Man could magically restore her face, which had been horribly mutilated after the spell allowing Mr. Punch to possess her collapsed at the end of Midnight Riot*.
But is it as simple as that?
Lesley May had been Peter Grant's Police Academy classmate and the closest thing he had had to a partner before Nightingale recruited him.
Also during Midnight Riot, Peter and Beverly Brook begin circling each other, but they don't become a couple until Foxglove Summer.
During Moon over Soho, Peter meets with Lesley and has an instinctive reaction to her disfigured face.
This is also where Zachary Palmer enters the picture. Zach is able to see past the disfigurement, and we discover that he and Lesley are having a relationship during Whispers Underground.
But still, at the end of the 4th book, Lesley joins the Faceless Man in crime.
So I guess the question sums up to:
Was Peter's rejection the primary cause of Lesley's betrayal? Was Peter her first choice, and did she think that by restoring her face, that he wouldn't reject her any more?
* I will use the American title to distinguish the first book from the entire series, and also because that was the title I read it under.