Unfortunately, nobody says you get to have a delightful day at work every day, or that your boss won't give you more work than you think you can handle. While the situation you are describing may be unpleasant for the employee, it may also be perfectly legal.
Employees often throw around the words "discrimination" or harassment" but may not know what that word truly means in a legal sense.
Managers need to know that discrimination is illegal only if it is based on a group, class or category specifically protected by law, or if it is done in retaliation for exercising a legal right such as filing a workers' compensation claim. For example, demoting an employee because the employee has exercised their right to take family leave to bond with a new baby would be against the law.
On the other hand, managers frequently make choices based on other factors, such as seniority, job department, or work performance. These common decisions may be perfectly legal. For example, a company could offer a better benefits package to employees who work in its office compared to what is offered to its warehouse employees, or allow a more relaxed dress code for the warehouse employees.
In each instance, a group or class of employees or applicants is being treated differently than another group or class, but because the distinction is not drawn based on any category protected by law, the "discrimination" is not illegal.
The issue at hand for the employee who is complaining of discrimination because she gets tough assignments and has a sarcastic and rude supervisor is to find out why she thinks the supervisor is treating her that way. Unless the employee claims it is related to her race, sex, national origin or some other characteristic protected by law, or unless the employee claims it is retaliation for having exercised some legal right (such as reporting a safety violation to the Occupational Safety and Health Administration), there is no illegal discrimination. More often, the employee will say something like "I just think he doesn't like me."
Also remember that, under both state and federal laws, employees have the right to discuss their working conditions, to talk to other employees about the terms and conditions of their employment, and to try to improve working conditions. Employees who complain about working conditions should not be retaliated against. Moreover, look closely at the complaints to see if there are any potential Labor Code violations (such as not being paid overtime for long hours or being forced to work off-the-clock). Employees also have the right to complain about violations of wage and hour laws.
Regardless of whether there is unlawful conduct, an employer would be wise to look into the supervisor's behavior and determine whether the company would benefit by providing the supervisor counseling in more appropriate personnel management techniques. While the supervisor's behavior may not be illegal, it certainly does not create a productive work environment and could lead to meritless, but expensive, legal claims.
You can find more information in the Discrimination and Harassment sections of the HR Library.