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.
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.
Each of us gains our own life experience, including in the field of gambling. Not all people who are interested in gambling become addicted to it.
I completely agree with you that prohibitions very often not only have the opposite effect but can also cause a loss of trust between parents and children. And if OP has the time and energy to monitor his daughter's gambling, he can control her activity and gently, without damaging their relationship, lead her to understand that she can replace gambling with a more exciting and less risky form of entertainment.