Mr. Mendez is a man who made a deal with Shadow Company to buy heroin from them for profit.
Mr. Mendez was entering Shadow Company's undisclosed hideout (which was a private casino and club) and was patted down and searched by Mr. Joshua's henchman Mr. Larch, Mendez saw it as a waste of time, saying he was not armed and went through already, but Joshua insisted he'd go through it again.
He then meets the leader of Shadow Company General Peter McAllister and arranges to buy some heroin from them. Mendez tells the General his mercenaries are wackos and he is insane. The General asks Mendez if he smokes, to which Mendez says he does, then he asks for his lighter. Mendez does not hear him at first, and angrily the General asks again and Mendez grabs his lighter out of his pocket.
Once he did, the henchmen grab him and hold him still, and the General uses his lighter to show Mendez he means no funny business by demonstrating it to him on Joshua. Activating it, the flame has contact with Joshua's arm; Mendez is disgusted by it and tells everyone they are sick. General McAllister asks him if he still wishes to do business with them and make a purchase, which he replies yes, knowing now that they really mean business and no funny stuff. Then as he is escorted, the General tells Mendez not to mess up with the deal of the shipment or else he will have to talk to Mr. Joshua.
It is unknown what became of him afterwards as he is not seen for the rest of the film thereafter. It can be assumed, and even implied that he was arrested due to his association while making a purchase with Shadow Company.