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.

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.