Forced to leave the company due to raising complaint against misbehaviour of a senior employee

Hi, My friend raised a complaint against a senior for misbehaving in office on regular basis. But no action was taken against him. Few other colleagues including females who were also suffering from similar issues filed a written complaint against him and in return top management assured of meeting everyone. Meanwhile company asked my friend to sign a performance improvement plan even when there were no performance issues. They tried to make a case against him. When he refused to sign they forced him to resign. What should be the next course of action ? Where can a complaint be made for inaction against that employee and mental harassment suffered? It's a Gurgaon based MNC.