Create an emergency call list of family and friends who could offer refuge for your pet and step in to foster temporarily in case of emergency. Remember to make a hard copy of this list that you can keep in your wallet or glove box, so you're covered in case your cell phone dies. In addition to finding out if your friend is willing, review other factors such as distance in advance. If she lives far from you, could you set up a meeting point halfway in case of emergency, to cut down on transportation time?
Consider whether your friend already has other pets and how your own might interact with hers. Additionally, you might want to establish an emergency savings account that you can dip into in order to alleviate any financial hardships for taking on a foster role, or even provide her with access to this account.