A few things to try
Sit in bath tub, let her pour water on your hair
Sometimes shower without the soap, just rinse with water to confirm if it fear of water or the soap
It's surely changeable. I use to be a child care teacher and I manage to coax a child in the same situation in2-3momths.
The mom is very surprise too
You must also be calm and let her know you can take control of the situation
If you feel she will be scared, she will be. Good luck!
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