I do not have a pattern, but my suggestion would be to buy a plain cotton triangle bandage and use that as a muslin type base to piece onto. This should give the proper size.Use a light or flannel batting, and if you leave the batting out at the tips of the triangle, you can then leave the main fabric and muslin to tie the corners. The other corner can be quilted, as it is pinned typically, if you have the time and ability to custom fit on her, you could sew velcro type fastenings so she could adjust it one handed as needed. You would need to bind it using this technique. Also, depending on how much warmth she may want, you could use flannel as a backing on the inside. If you decide to use a backing fabric, you could do as "envelope", where you sew the right sides together and turn right sides out through a small opening you left unstitched. Hope this makes sense! I am sure somewhere on You Tube you can find a first aid video to show how to tie a triangular bandage into a sling. It's fairly easy.
Just a few thoughts, hope it may help! Sorry to hear your friend is in pain. It's great that you want to help. Good luck and let us know how it works, whatever you do, someone else may want or need a similar item.
GEMRM