Procedure to file a defamation case

I belong to Muslim Community. My elder brother is employed in a PSU. His wife filed a writ petition against him u/s 498A, 406 in the year 2005 in Tis Hazari Court, Delhi. In the year 2006, my brother divorced his wife in accordance with Muslim Shariya Law. She also accepted the divorce voluntarily. In January 2018, the Hon’ble Court dismissed her petition due to devoid of merits. In April 2018, she has filed an appeal against dismissal of her petition in the Session Court of Tis Hazari, Delhi. My brother intends to file a defamation case against his ex wife for filing a false case against him in Tis Hazari Court in the year 2005. Please clarify whether my brother can file a defamation case against his ex wife now or he can file the defamation case only after disposal of the appeal filed by his ex wife in the Session Court. Please also intimate what is the time limit for filing a defamation suit in case of my brother.