I would advise this guy to quit gambling and save his mother's psychological health, or to find a part-time job and spend the money he earns as he sees fit. In this case, the guy will have a job and his mother will not worry.
I agree. Even he's already in legal age to gamble, the point is, he's still depending on his parents financially and the money given is meant for his studies and not for leisure.
Thus, it's normal for parents to be worried if they became aware that their son is into gambling. Therefore, an advice of looking for a side job is better rather than relying on his mother's given allowance. This way, the son can have a budget if he wants to entertain himself through gambling.