It is better for you to continue to warn your daughter to start reducing or even stopping otherwise you will lose more.
Well, his daughter already learned to gamble so it's useless to make her stop from doing it. Forcing her to stop might be more bad as it will just entice her to gamble more.
preventing her will not do anything she will keep coming back for more since you already showed her how and worst thing is you are not even able to monitor her since she will gamble behind your back
What op can do is just guide her daughter and watch closely her gambling activities. Make sure she doesn't go into addicition like him. Maybe it can help if he will tell his story to his daughter about how he got into gambling addiction and that it didn't do good to him.
that indeed would be the right thing to do and let us just hope that she takes the story to heart and learns from it before it is too late