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 child or a teenager stealing money from his father or mother or anyone around him/her is a question mark on the upbringing of the parents in the first place. So the first thing they should do is question themselves about how they have raised their child. Once they do that, then they should think of a way they can handle the situation and they should make sure that whatever they do should make the child understand, it shouldn't make him too much frightened but he should learn his lesson so that he doesn't repeat the same thing again or do anything that is considered bad.
A sitting with the child should be okay but it shouldn't be full of affection and encouragement to make the child think that he can do whatever he wants because the elders won't mind or do anything at all.