It's easy to say this but in reality, it is almost impossible to convince the gambler that he is addicted to gambling and even more different to ask him to quit gambling. The gamblers don't care what is good for them in the long run, they only see the present and want to earn money through gambling.
I don't think it can be easy for anyone to quit gambling. One situation can be that the gambler loses everything and become out of cash, only in that case he may quit as he has no other option but other than this, I don't see gamblers quitting.
If it's during an early stage when the person just started gambling and realised that they were getting addicted to it, it can be very easy to just quickly tack on a U turn, but once it gets out of hand, it becomes difficult to stop it, like you said. But that doesn't mean it really can't be stopped; if the person in question really has the intention and the decision to stop gambling compulsively, then they can do it on their own or even seek help on how to stop it. Based on what @Wiwo said, it's true that if a gambler is greedy and sees gambling as the only means of earning money, they might get stuck and addicted more quickly.