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.
My vision is different. The only one to blame here are those who raised and educated that kid. If he was taught properly about what crime is and how to get money at his age, I think he would not acted like that. Not saying that kid is completely innocent, but there were reasons why he did this. It is not like he learned how to steal himself. Someone showed, told, explained, or lack of explanation what is good or bad had lead to such outcome as stealing from family.
Why is there a need to blame? We don't actually put a blame on someone if the child grow up the way they are right now. I mean the parents and the educators have done their part for sure. Now, as the child grew up, he would learn a lot of different things, bad and good from his friends, acquaintances and classmates. It is inevitable not to expose children to what kind of world we are living for, and we are not always there to guide and support them in every step they make, because we also have responsibilities from our businesses and jobs to keep our family financially healthy.
Now, if your kid gambled and non of from your family is a fan of gambling, then he has learned it outside. Now, again we will not going to assume that the parents have not done anything for their child to get rid of gambling totally. The kid's decision and mindset is out of our control. Sometimes, children are too stubborn or to secretive for you to learn that they are actually pursuing bad things in their early lives, because again, we are not there to guide them every step of the way.