turn king's/queen's evidence
To provide evidence in court implicating other parties involved in the crime for which one has been charged, in order to receive a reduced sentence or to avoid prosecution altogether. Primarily heard in UK. We're leaning on him pretty hard, so we think he'll turn king's evidence and finger his accomplices.
turn King’s/Queen’s ˈevidence
(British English) (American English turn State’s ˈevidence) give information against other criminals in order to get a less severe punishment: One of the gang turned State’s evidence and identified at least three others involved in the fraud.