Well I'm not trying to judge him but for a father to allow his underaged kid to gamble is very wrong even though she's very close to the gambling age but he's supposed to allow her get to the required age before doing that if he could allow that it means he'll likely allow her do other stuffs that's not required of a minor which is very bad. How will he feel is she ends up becoming addicted at that tender age.
If the child was not his, it could have been said that he allowed the girl to gamble even as she's not of age, the reason being that it's not his daughter, but this one. Since the girl is already gambling, even if he tries to stop her, the girl can still gamble behind his back, but there is no harm in trying. And given her this kind of advice, rather than to embrace and support her in that which she's doing, it's not just about gambling; such a decision will have a great effect on the child.