i don't know the age or health status of your daughter but shower-fainting is not that uncommon. it either is orthostatic hypotension (lowered blood pressure when she is in hot water and all of her blood starts pooling toward her legs/feet) or perhaps even a "vagal response" where the blood pressure is lowered when she holds her breath (washing hair and face). a few suggestions: shorter less-hot showers and get a shower seat so she can sit. also make sure she is not dieting and throwing off her electrolytes.
my daughter faints usually happens when she is taking a shower. tests did not show anything to explain it.