Dear professional nairalanders, I would like your advice on this issue on behalf of a friend.

Last year, my friend was employed to render professional services to an oil company through a third party. I connected him to the job. He works as an NDT professional and the main contractor and third party are based in Lagos.
He was issued an employment letter that stated he would be paid monthly. Initially, the agreement was honored, but later, payment became erratic. He is now owed 800k for the past 6 months. The company does not reply emails and pays in dribbles as they like. At the rate they are paying, they will complete payment in a year. This is clearly not acceptable. The fellow has many responsibilities to shoulder.
The contractor and third party no longer work together and this is given as reason for delay. This should not be my friends problem : he had an agreement with the third party to pay him not the main contractor. They do not deny that. The third party that engaged my friend is a big and solvent company still doing business.

How can he efficiently collect his money ?

Lawyers and experienced professionals please chime in with your thoughts.