Some people like to make fake calls. They enjoy while troubling others. Ask your friends not to worry about it. This case will fall under cybercrime. They should ask the caller for details. It might be possible that the caller has the wrong number. Your friends should ask the caller to give the details of that girl or the incidents that happened with her. They can assure the caller that they are innocent but can help him/her. If the caller would be genuine, she/he will agree to give the details. Or else he/she will hesitate.
If possible tell them to handle it by themselves. Scare the caller by saying that they will file a complaint against that number under cybercrime.
This might help them.
If they further get a call then ask them to talk to their parents. Ask them to take advice. Ask their parents whether they should involve authorities in this matter or not. And act accordingly.