When you find out Shaggy, can you let us know what you find out? She is a very pretty girl, love her ears! Looking at her shoulder, if it isn't a soft tissue injury, could it be what is called a sweeney shoulder?
Arthur Veterinary Clinic: Sweeney in Horses
Glad you found her, if it were me, I would of put a small trough or something she could get water out of, she needs some groceries for sure but hopefully you will find something out. Have you called the local animal control or vet offices that deal with large animals? Sheriff's office? Ask if anyone has lost a young grey mare.