Forcing To Sign NDA

Sir employer is forcing to sign non disclosure agreement saying otherwise file a case against my friends. Nowhere is written in service agreement emplyee needs to sign it after resignation. Pl advice

If your friend has already resigned and such resignation has been accepted then the employer employee relationship ends there. After cessation of the relationship, employer cannot ask you to do 'anything'.
On the other hand, you can write to your employer and ask him to stop threatening you and forcing you to sign NDA since you are no longer in their employment. A copy of such letter can also be forwarded to the police station as abundant caution.