Sir your question is not very clear, id a complaint of sexual harrasmaent was filed against a person which has now been proved and he is guilty, then he has no ground gor filing a defamation case.
Criminal defamation is filed under s499 of Ipc and civil defamation is filed on civil court to seek compensation, however in your case that person has no right to file a defamation case since he is guilty as in order for a defamation case to hold up, it must be proved that the defamatory statement was untrue, and caused damage.
Let me know if you need further assistance.
Regards