How can you complain about being wrongly denied boarding and demand compensation?

An African friend of mine was traveling from Dar es Salaam to the UK (Manchester) and had an "Indefinite leave to remain" visa. His wife and children live in the UK.
He was flying on Turkish Airlines and was denied boarding the plane to Manchester at Ataturk Airport Istanbul when he was at the gate. The Turkish official would not explain the reasons or respond to questions of my friend but said the documents were not adequate and kept shouting "don't talk".
After 6 stressful hours and a lot of unpleasant chatting to other officials, my friend begged the police to help him. They organised a flight back to Tanzania using my friend's return ticket (which should have been in Jan 2017). He had no food or drink until he arrived back home.
Two days later he did fly to Manchester on another airline (Etihad) using the same documents which where accepted by both the airline and the immigration.
How can you complain about the behavior of the offical at Ataturk airport?
How can you claim compensation for the ticket if my friend has wrongly been denied boarding?
This cost my friend £800 and I'd like it to be investigated.
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How do I claim compensation for denied boarding?
Accepting the airline's offer will mean that you voluntarily agree not to fly on that flight. According to denied boarding rules, you can only claim compensation in cases in which you were involuntarily refused access to the flight. This is why, in most cases, we would recommend that you refuse the airline's offer.Can I get a refund if I was denied boarding?
Where involuntary denied boarding occurs, your air carrier must offer you the choice between; re-routing as soon as possible; re-routing at a later date at your convenience; and. a refund of the full cost of the unused flight ticket.How do you handle denied boarding?
If an airline denies boarding against a passenger's will, it must:What happens if an airline refuses to pay compensation?
In the event an airline refuses to acknowledge your claim, you need to seek legal advice or file a complaint with the airport's authority. However, you need to understand the reasons for the denial of the claim. Sometimes the extraordinary circumstances are legitimate.HOW TO CLAIM COMPENSATION FOR FLIGHT DELAYS| AIRHELP
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Mathias Reding, Nothing Ahead, RODNAE Productions, alleksana