Why is Stephanie compliant in Steyn's plan to destroy Pieter in the novel, Too Late the Phalarope?

Stephanie, threatened with having her child taken away from her, seeks out legal work, but is unable to hold onto such a position, so she returns to earning money illegally. Faced with the reality of losing her child, she goes along with Sergeant Steyn's plan to destroy Pieter, and betrays him.