Senior Citizen- Credit card fraud, or just gullable?

My Grandmother was asked by "a friend" to get a credit card in her (my Grandmother's) name and allow her to use it 'for emergencies'. The other woman has since charged $18K+ to this card, over a span of about 8months. She has been collecting the bills from my Grandmother and just pays the minimum amount due every month. The family just learned of this recently, and have since found that this woman borrowed cash and also had Grandma co-sign on a car loan. She believes that this woman is a friend who has fallen on bad times. Is there anything we can do to stop this woman from using her card without my Grandmother being willing to cancel the card or take legal action? Can legal action be taken if Grandma gave permission for the card to be used?