When is it inappropriate for a daughter to see her dad naked?

At what age is it inappropriate for my daughter to see her father naked? My mom thinks its inappropriate for my 4 year old daughter to sit in the bathroom with her dad when he showers or baths (door is always open, that’s just how our whole house is.) She likes to talk with him, sometimes she will wash his hair. She likes to take care of her dad. When he is done she’ll brush his hair. She is really comfortable with him. He doesn’t go out of his way to cover himself but he also doesn’t sprawl out when she’s in the bathroom. Anyway she does not ask questions about his body but this has been the “norm” in our house. Just looking for opinions.