Boris owned a series of cars and even used his money to buy a famous soccer team. Boris also had a girlfriend who he ignored constantly (possibly due to stress of his deals and his soccer team) so he was usually found in his hotel room sitting on the couch watching TV while his girlfriend hung out at the bar, bored.
Sydney and Nadia found out that he was making a deal involving Black Thorine, a powerful explosive, and managed to figure out where his next deal was by cloning the records in his PDA. The next day, Tambor did the deal with a man named Leo Orissa, who was head of a Russian Mafia Organization. The deal was going down okay, until Leo said that he planned to use the explosives to take over Tambor's business. Tambor, thinking he was joking, simply laughed at him...until Bridget took out a concealed pistol and tried to kill him.
After Leo and his guards dispatched of Tambor's guards, he tried to flee his yacht, but wound up getting caught by both Bridget and Leo. Both of them quickly shot Boris in the chest several times, hurling his body into the ocean.