Don't intervene, you might end up putting yourself in a situation where you might start regretting your actions...One of my principles Is that I never get involved in other peoples family issues, even though it might seem like you need to speak up and tell his daughter what is actually going on that is not going to be a good move..sooner or later she is going to find out, things like this don't stay hidden for long
It can be a problem if we interfere in other people's family matters, but I feel sorry for his daughter who has to bear the consequences. If that happened in my environment, I would help her, but I wouldn't tell her father about the problem because, as you said, sooner or later he will find out, and hopefully, he can accept this patiently. What I fear is that this addicted father will abandon his responsibilities as a father, focusing only on his own gambling desires without thinking about others, including his own daughter.