That friend of yours is very lucky, huh?
How does he explain winning/hitting 10 consecutive predictions without any losses along the way? Anyway, I'm not asking your friend, since that's not the focus of the discussion.

In my opinion, I don't think this casino's practice of keeping the customer's money is fair, even if this is in the contract, the most the casino could do is limit that user's account/IP, reduce the amount of bets and so on. Are you going to keep the client's money and all of it?

If I had a casino I'd not choose to keep the customer's money, although your friend is having a lot of luck, one day he will lose, as will the other players in my casino if they are making a profit for me with their loss, my casino would remain viable.

But if it's the casino's rule to ban users who always win or keep their entire bankroll tied up for X days, there's not much that can be done unless the user is willing to take the case to court, but if the casino is online and from another jurisdiction, it would complicate things more.

Not to mention that this can create a headache for the casino, as the user can expose it on social media, if it is an influencer the casino is screwed, the chances of losing customers and potential customers increase.