Unfortunately, those who are very much hooked on gambling will ignore every sign and will just continue to gamble, even suggestions coming from friends and relatives, they will only realize it when it is almost too late that they almost lose everything, this is when they start to realize that they are a compulsive gambler and needs treatment.
I keep saying this but this is the reason you have to be aware of their addiction and treat it as any other addiction. Not with stigma and negative emotions but with supportive ones. And encourage them always to reach out to you.
In terms of prevention, buddy system! Have someone to monitor you, and you monitor someone, together you help each other prevent over compulsion,,, win win.