The Folly

I find it really difficult to make predictions, given that Aaronovitch is a complex and inventive writer enough that one cannot really say "oh, but Lesley was already brain-washed once, that won't happen twice" or "she'd never truly betray her mates for getting her face restored". He might do it and make us like it, in the process. However, I do think that Peter is not the most reliable narrator when it comes to Lesley. Too much emotional involvement, too much of seeing what he wanted to see.It says something about Peter (and he's admitted to being shallow before) that his immediate conclusion is that Lesley went with the Faceless Man to fix her face.

The scene when she left is actually a bit more ambiguous than it seems. Did Lesley save Peter's life? Probably. Did she save it by tasering him as well as after when she stopped the Faceless Man from kicking him senseless? Don't know, because Peter does not know. If she had not tasered him, what would have happened? Two dead apprentices?

I hold out some hope that Lesley and Nightingale planned something together, as there was certainly time enough for that.

But I'm also wondering if Nightingale might have set her up to betray him. If she passed, she'd have proved her loyalty. If she fell for the Faceless Man's promises, Nightingale would control exactly what information she went in with.

The Faceless Man may also be a bit more clever than Peter thinks. What better way to ensure that Lesley cannot go back, than making it seem as if she's been aware of or an accomplice to murder?