I don't know what you can say to this person about the daily washing. If telling her that daily washing of very curly hair isn't necessary, and that it's drying out her daughter's hair, and causing more wear and tear by doing so isn't enough of a reason, I'm not sure what would convince her to stop.

However, maybe we can minimize the damage she's causing by washing every day. Try to convince her to just wash her daughter's scalp only, and with a mild shampoo. It's the scalp that gets oily.

And when she washes, she should never ever pile her child's hair on top of her head and rub it in. That's horrible for our hair. She should let her daughter's hair hang down and smooth through the entire process, wash just the scalp, and let the shampoo run down the hair as she rinses.

And she needs to always always use a good conditioner to comb her child's hair with every single time she washes it.

Also, no rubbing her child's hair with a towel. That is also terrible for our hair (well, anyone's hair, actually). If needed, she could gently squeeze sections of her hair in the towel. But to be honest, if she leaves it wet, it's easier to comb wet hair. She could just drape a towel over her daughter's shoulders to catch drips. I don't even use a towel to dry my hair at all.

I don't know if she knows this, but when she combs her child's hair wet (and with conditioner in it), she should comb in small sections, and should pinch the child's hair between her fingers, in the area between her scalp and where she is combing to minimize the terrible pain that occurs when curly hair is combed.

She can check out Tips for Little Ones for more ideas.