I can think of several options how this scam works:
1. He does nothing, but still has 50% chance that user wins and will pay him.
2. He is in contact with casino websites,some sort of referral program and gets bonus on money his clients lose. Furthermore,casinos may even be aware of his scam, and let user to win first time(if the amount of money is small) and then when stakes will be higher casino will win.