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.
It seems that is what is on my mind that he really needs to call the police to advise his child too and undergo a small punishment to have a deterrent effect, because basically a bad habit that is often carried out will have a bad impact in the future, the child could have dealings with other people. someone he really didn't know then stole from his house while at home the person had a gun and saw his child steal and then shoot him in the leg. Maybe it would be worse if he continued to imagine the bad impact in the future.
So the mother's attitude is not wrong, after all, whatever the circumstances, never try to gamble with the proceeds of debt and secondly the proceeds of stealing, both of which are not suitable for gambling because there will definitely be bad consequences in the future, it is better to gamble with money that is ready. To get rid of it is not to lose money by borrowing and stealing, if you don't have money it's better to look for another hobby that doesn't require money, but if you have money there's no problem gambling, sometimes people push themselves too hard to keep gambling no matter where the money comes from.