This is a fine hair to split, but I think it's an important one: Qrech is a prisoner of war. He was an officer in the army of a nation that seeks the complete destruction of the Empire, and the only reason he has never directly contributed to that goal is that he was instead contributing to one of the Skavens' simultaneous wars with pretty much every other polity in the world. The work he does for Mathilde is voluntary, not onerous, and performed in exchange for comforts and luxuries; his basic needs would be met whether he cooperates or not. Modern perspectives can point to ways he could be better treated, but to an in-universe perspective Mathilde's treatment of Qrech has been incredibly generous, to the extent that if it were not for the intelligence she has been able to extract from him by doing so, some might start to question her loyalties.