Is it illegal for the ex-wife to slander the incumbent under the guise of fortune telling?

Break the law. Defamation of others may violate the Law of People's Republic of China (PRC) on Public Security Administration Punishment. If an ex-wife slanders her present job on the pretext of fortune telling, it is a fabrication of facts and false accusation against others in an attempt to subject others to criminal investigation or public security administration punishment, the perpetrator will generally be detained for less than five days or fined less than 500 yuan.