We stopped washing L's hair (granted, he's a boy so maybe it doesn't matter so much). We did that after an incident at daycare when he was around this age when he held his friend's head under the water in the bath. Everyone was shocked that my sweet boy would do something so dangerous and when I asked why, he said he was washing the boy's hair. Once we understood how HE was experiencing the hair-washing process, we stopped forcing it on him and actually, he had a healthier scalp after we stopped than he did before. He now washes his own hair of his own volition and reasonably regularly - that comes with time, IME.
Can you chat with her about ways she could wash her hair and be happier about it? Maybe some goggles? For L, it was not being able to breathe freely when water was going over his head and running down his face that upset him and he felt like we were drowning him.