From what I read in the article, the child is not directly paying for the rent; the mother is just asking her to pay that amount every month in order to support the family. Aside from the discount that they are already getting from her workplace, it's a small amount compared to what the child earns in a month, and if I'm in her position, I will not complain about it. 
 
In some families, if the child is already earning a salary that is enough to cover family expenses, he or she doesn't have to wait for the parent to request money before they can start assisting the family. Some will even ask the parent to stop paying rent and use the money for other things. It's just a matter of self-decision. If you are to talk about legality, the girl is off-aga and is entitled to pay her own bills where necessary.