Is she trained to stand tied? Is it that she's afraid of the hose?
I would start with desentizing her gently to things normal in a bath. Shampoo bottles, hoses, etc. For now, I would bucket wash her carefully. Try keeping her distracted with something like a net full of hay or some grain.
It will take time, but she will learn to trust such things as scary spraying water and sudsy bubbles.