In general, there is no "reasonable expectation of privacy" in a private employer's workplace (except in a rest room). Employees of state or federal agencies have marginally greater protections.
So, if an employee is conducting personal business on the employer's premises, and the employer has not expressly granted the employee privacy rights, by way of the employer's policy manual/handbook, or some other express written contract, then a co-worker can surveil another co-workder, and the only recourse is to ask management to prohibit it -- or, quit your job.
That said, there are laws against harrassment based upon discrimination (race, color, religion, nationality, gender, disability, age -- others may be applicable by jurisdiction). Also, there are laws in some jurisdictions (e.g., NY "evesdropping law") which can sometimes be brought to bear on the sort of situation that you complain about, depending upon the precise facts involved.
But, as a general rule, if an employee wants privacy in his/her communications, the only way to get it is to leave the employer's premises entirely.