That's really unfair, and I'm sure he gambled at a casino that has a bad reputation, and I would ask if there are measures to just make sure that it is a trusted casino by looking at its reputation first before he finally decides to bet at the casino? If not, then that's the consequence that we don't get justice when we are really lucky by getting a lot of lucky wins, even though the casino promises to pay it but if it takes a year to be able to cash out the winnings then I think a gambler will still feel disappointed.
On the other hand despite everything I think it is really a very concerning situation and surely he is very disappointed with the service of the casino, because the casino should behave fairly which is simply that if there is one gambler who has won a lot of consecutive wins then the casino should have a good responsibility to pay the gambler according to the amount won and at the right time without any delay in payment, especially in that case he did not commit any fraudulent acts at all. In conclusion, I think this incident can be used as a lesson for gamblers to first check the reputation of the casino to ensure that you are involved in a trusted casino.
I also don't think it will happen in big casinos like stake. And the amount was not so big either. So why use a small and not reputable one if you don't want to have issues like this, I am just assuming a small one. The OP said, he is a friend of his. Since he is promoting stake, why not introduce him to stake because this casino can handle huge amount of money. Just look at the amount Drake is betting on this site.
There is nothing else I can think of other than concluding that it is a bad casino that is not reputable or that does not have good responsibility in running its gambling business, because before an incident like this happens, or before someone manages to achieve a lot of winning streaks then they should have prepared funds to pay whoever wins, and also I think at that time there must be many other gamblers who have lost at the casino so I think there is no reasonable reason to delay the payment... On the other hand, yes, it is true as you say that if it is OP's friend so yes I will also ask why he doesn't suggest getting involved in the casino promoted by OP, because so far I have never had any problems when betting on stake.com, especially in terms of deposits and withdrawals, unless we do some mistakes or breaking the rules then that is another thing.