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I'd go for Cary's suggestion to the letter writer. Run away!
The father is responsible for the daughter, the letter writer is not. But if she stays with the father as a married or de-facto partner, then she would be putting herself in a situation where she would be expected to be responsible for the daughter.
If she can't handle the 'messed-up' daughter then she should leave. Perhaps the girls father will meet someone who may be willing to look after the girl... who knows? But staying would eliminate this possibility and simply create an unhappy and unsustainable family unit.
Whats in the best interest of the daughter is to have a stepmother who loves her.. if the letter writer feels she cannot be this person then she should simply leave now.