Katherine has two employees who have never seemed to get along.

Katherine has two employees who have never seemed to get along. One of the employees has a history of being vindictive and manipulative but never in an obvious enough way for Katherine to have sufficient proof to reprimand her in writing. One day, this employee comes to Katherine's office to report that she saw the other employee, who has an exemplary record, taking drugs from the supply cabinet. How does Katherine react to this situation? What steps should Katherine take from here?