Breach of artist agreement- payment not cleared even after 6 months

I am a lawyer but have never handled litigation cases single handedly. I am trying to help out a friend who did a freelance gig with a production house as a side actor. After completion of shooting, it has been almost 6 months that the producer hasn’t paid him anything.

The person who was the point of contact claims that he left the production company and that almost all the side actors who did small roles were not paid their dues- all of them have been following up on him.

There is the artist contract which mentions that the actor is to raise invoice and then within 30 days they will be duly paid. Despite raising invoices, there has been no response.

My friend has emailed the production house with invoices, called them numerous times - no response. He has even tried to visit their office for that but still no help.

I suggested to send a legal notice since it has been a good 5-6 months from the date of due payment, and technically he is entitled to some compensation for this harassment and unexplained delay.

Any advice or help or inputs for this? What can I do? How can a legal notice make them comply? What points should the notice ideally have?

Please note- I do have an AIBE license just need some expert advice before proceeding, consider a junior taking up their first ever solo case :)