First of all, there's no such thing as free... Your mom paid for or is still paying for the iphone she got... it's incorporated with the bill that she's paying. If she did not avail of it then she would have extra credits for services which is usually 25% to 30% of the cost of the monthly service fee. It used to be that Globe would give bill rebates if one did not avail of any gadget but they got wiser by forcing us to subscribe to their services instead.
For your concern, change the apple ID and/or change the billing details and remove her credit card. Be careful about these apps and just get the ones which are free.