Without wasting of much time the mother should relocate the environment they live in and block every means of having access to gambling by her son, a teenager needs to focus in his studies and not gambling, advise her to discipline the child now before things got escalated out of hands, if he have no money to use for gambling and he's not working to earn money, he will definitely begin to think on stealing for has access to gambling.
What if he's lying, how sure are you about what he's telling you, children of nowadays are very smart in doing wrong things, yours is to better advise him to face his life career ambition and study hard to turn whom he would always wanted to be in life, advise his as a brother.
Even if this be the case, it's not still good enough, he can have savings and have plans for himself than using his money on gambling at his age.