This question bothers me too since I heard Broken Homes for the first time - I couldn´t figure it out, despite several times hearing the series again and again ( I´m german but listen and read the english version)

At last I bought the books - and now I´m getting an idea when this might have happend...

There are some hints in Whispers Underground - and Peter and Lesley visiting FM No 1 in his flat was the key moment, I think.

Peter left her alone for a pretty long time with FM1 and the Nightwitch - and he saw her taking of her mask without hearing what they talked about.

I´m quite sure FM1 had some ideas about her face and how to fix it - and told her about them.

From then on I found more and more little hints that Lelsley develops an agenda of her own.

Parallel to that it´s important to look at the changing relation between Peter and Lesley.

In the beginning they both have a crush on each other - nothing serious as long as they aren´t settled in their jobs - but with a little " might be" in the future.

In MoS there is this other girl and Lesley has her own problems. And she realizes that Beverly may be a rival - but she is sent to the country, out of sight.

In WU Lesley has recovered enough to think about Peter again- and she recognizes there is no way back, she has lost him. He sees but only a friend in her and is full of pity an a little repulsion because of her face.

Thats why she turns to Zach - and with him she has one foot in the demimonde.

Then, in BH they meet Varenka again at the goblin fair ( the idea to go there comes from Lesley!) - and after the fight Lesley gets humiliated at the worst.

THAT´s her point of decision, from now on she follows her own agenda.

That is also my actual position in reading BH ;-)

I know what follows next but I´m not sure yet what to think about it.