I don’t think the mother is mad about her son gambling, just the way he got the money. Stealing is a crime and if unchecked he could develop a habit for it and steal from someone who is not as sentimental and would very well call the cops. I think the mother is trying to prevent such an occurrence from happening in the future, the OP doesn’t say the age of the boy, I can only assume he’s a teenager. The family should have a sit down with the boy and let him know the consequences of his actions.
A mother will be angry when she discovers her son stole her money. Stealing is a crime, but a child must be given understanding so that he can understand that stealing is a bad act and he can be punished, and the worst is that he can go to prison. With the understanding given by his mother, father and other family members, the child will understand and will not repeat his actions. He will not steal money from his mother or other people because he already knows that stealing is bad and can be caught by the police. He doesn't want to experience that and will stay away from it. A mother will prevent this from happening again and always look after her children well.