I think it's credible that being viciously raped could bring about that change in a person, though.
I agree and let's not forget this dialogue from episode 2:
Yvonne - ‘Can’t you warn him off for me?’
Costley - ‘Nothing would give me greater pleasure. Don’t tell me you wouldn’t like to put the wind up that pathetic piece of s**t.’
Yvonne - ‘I would. I want him to crap himself with fear. I want to feel half as terrified as he made me feel'.
In the book she says: 'I want you to smash his face'. But in the programme she says 'I want you to smash his fucking face in'. I think the swearing that was added in this scene by the screenwriter is a pretty clear indication that she understood from reading the book that Yvonne knew exactly what she was doing.