If we were talking about a stranger stealing that money from her I would agree with you, because we must do what we can to protect our money, however it was her own son who did this, meaning that a person she trusted, broke that trust by stealing from her own mother, and while the amount was small, what can be more low than stealing from your own mother? So I would not blame her at all for this situation as it is clear that it is the son that is at fault here.
There are many ways to stop strangers from stealing your money and treat him harshly for his act but no one can do this with his own son. A person will just tolerate if his own son start to steal his money but will not do anything harsh with his son.
It is important to be noted that a gambler always force his mother to help him in getting money but he never ask about it from his father because a father can understand that his harsh words can change his son so he will not talk him sweetly but mother on the other hand will wish that his son never see difficulties in life so she will surely help him in bad activities too. But there are only few mothers who change her behaviour for making her son a better person in a society.