Did you see this one?
https://sewingforlife.wordpress.com/...h-bag-project/
You should wait until she gets her crutches to get the exact sizing for the bag. You could also use any small flat bag pattern that you had. Make sure it fits within the crutch and doesn't stick out on any side. Attach Velcro to the top to hook it onto the crutch. Also need ties or Velcro for the bottom corners so it doesn't swing and interfere when she actually is using the crutch. Hope she has a speedy recovery.