If I where the woman, since the deed has already been done, and is obvious that the son have lost the bet, trying to get him refund the money will only lead to more crisis for both the boy and the entire family because he may be pressured to provide the money by all means and by that be forced into stealing from others to be able to pay back and in the process ended up in a bigger mess.
You are right, the OP said the boy lost the money to gambling, how will the boy be able to refund the money? The boy isn’t working yet, so if you can force the boy to refund the money, then he will definitely go outside and steal just to be able to refund the money, and if the boy get caught outside, it’s going to cause more problem, and it’s going to bring shame to the family. The best thing is just to warn the boy to stop gambling, tell him the necessary things which the boy is suppose to know about gambling, and tell him never to repeat what he just did. I think that’s the only thing the mother can do, she shouldn’t force the boy to refund the money.
I don't know what could have triggered the boy to act in that way, but most times, such behavior could be a reflection of something that could have generated from the parent, may be there have not been proper training for the boy and he may have been left to fend for himself all the while to have thought of gambling means he have been independent all the while.
Most of this bad characters are learn by children from school, and social media, since most of them have access to mobile phone, they can easily take their phone and gamble without even parents knowing about it. Some of them will even clear their browsing history, they won’t leave any traces that will make you even think they are gambling, you won’t blame parents always.