I get at least a few calls a week where an employee complains to me about being treated unfairly at workplace. these complaints range from being given more challenging and time consuming tasks than co-workers to being yelled at by the manager to being micromanaged and written up for every little thing.
My answer is the same every time: it is not against the law to do any of the above or to be otherwise unfair to an employee unless the reason for a different treatment is discriminatory or retaliatory. In other words, if there is evidence that the reason for favoritism is because an employee is a member of protected class (due to a disability, age, sexual orientation, familial status, etc.) or due to engaging in a protected class (filing a workers compensation claim, complaining about health and safety issues at workplace, filing a sexual harassment complaint, union activities, etc.) then the aggrieved employee might have a legal claim which can be brought in court.
Otherwise, if it is a matter of inter-office conflict which is not based on any discriminatory criteria, that employee will likely not benefit from having an attorney, and he/she should seriously consider looking for another job, if there is no one in the office who is willing and able to help. This is because the California courts have mentioned many times in the past that their job is not to enforce civility rules at workplace but to only remedy legal violations, such as discrimination, harassment, and retaliation.