It is a human nature to let go of a simple error if the result outweighs the error a million times. Obviously, the reason why the mother gets angry is because the son stole the money and lost it to gambling. In your given question, since the kid can give back the stolen money with interest, I am sure even stranger will forgive the kid and will receive the money with a smile. Probably that stranger can even hug the kid that gives back the money with huge interest. What more if you are the mother, for sure the kid will probably give you the whole winning amount. So is there a reason to get angry in such a situation that you asked?
It depends on the parent's perspective. Perhaps you believe that stealing money is a forgivable offense, or you are the type of parent who is quick to forgive their child for such a mistake.
The thing is the kid gives back the money he stole with an interest. The thing here is returning the money. As a parent, a simple theft can be forgiven, and forgiving does not condone the action but rather giving the kids another chance, plus the fact that the kid returned the money means he knows his mistakes and wants to correct them by returning the money.
However, it is not good to easily forgive a child for stealing money at a young age because it may lead them to believe that it is acceptable to do it again in the future, as they can easily avoid facing the consequences.
So what do you want to do with the kid? jail them or beat them up because they stole a small amount of your money? I think that is a harsh thing to do as a parent. Reminding/scolding them that stealing is not a good act is enough. Besides, if you beat up your kid as a parent, you will be charged with child abuse, and that makes the scenario worse.