I have a similar situation with ym big girl Dessi (rescued) and what I have done is exactly as stated above kept track of feeding and husbandry costs. As well as medical care given to the snake. If the young girl wants her snake returned she first has to prove she can and will be able to take care of it and second Repay at least a portion of the Husbandry/Food Expense and all the vet bills. So far she has not asked for the snake returned.
Hope this helps you out