Mary needs to make sure that a process cannot be subverted by a single employee. What security control can she implement to prevent this?

Two-person control schemes require two people to be involved to perform an action. This means that Mary can implement a two-person control scheme knowing that both individuals would have to be involved to subvert the control process. Biometrics will merely validate that a person is who they say they are, robotic sentries do not add any particular value to this scenario, and a demilitarized zone (DMZ) is used to keep front-facing systems in a zone that can be controlled and secured.