Can an employee be drug tested without notice due to a change in policy?

Question Details:

When a part-time employee was hired, he was told he would not be drug tested because he doesn't receive the same benefits as full-time employees. A new boss came into office and wants to only drug test that part-time employee. The employee was never told of any policy change. They gave him an ultimatum in office to either take the test or be fired. There was no warning, no informing the employee of any policy change subjecting part-time workers to drug testing. Is this entrapment or illegal in any way?  Is this a violation of privacy?

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