Tia is too... goal-oriented to have sent the Duke after him because of human trafficking.
This is what I think is happening.
This is Round One of Tia's power play to get the Duke on the imperial throne. They mentioned that the casino owner is one of the backers of Grand Duke Roygar. If you remember, he is one of the other option for emperor besides Tia's brother, but earlier Tia dismissed him as being too greedy for the position. Destroying the casino cuts off one of Royger's income streams and dirties his reputation, since the casino owner is known to be in his camp. And Royger is more involved then just skimming a little money off the top? (Which Tia would probably know about.)
Duke Evron will destroy him.
Duke Evron is a relentless guy, and he is going to dig to the bottom of this. Especially now we know they are traffickers, his honor would demand no less. So Tia set Duke Evron up. She's using him as a sword to cut down everyone in his own way to the throne.
Nice, Tia.