On one hand, its just her being nice and letting her child play online. On the other, its the child being an idiot abusing something he shouldn't have. I'll bet you the mother didn't even know of the market place, just thought it was to pay for online.
Also, have you ever tried to remove the credit card from the account? you cant. Watch, go log into xbox.com and try to remove it. You have to have ANOTHER on there to do it. And to cancel your auto renewal, you have to CALL. Its BS and chips
Honestly the bank should have warned her too