Black Thin Line On Nail?

Hello everyone. My name is Kate. For about 2-3 years, my sister who is 10 years old has a strange thin black line going down her pointer finger. Her nails have all these ridges.. The black line on her finger is not that dark but it is a light black. She wears nail polish often, so I wonder if that's it. I'll tell you some stuff about her that may help: she's very thin, she doesn't eat much, she only eats meat, she only drinks milk, and she's always stressed. Please help, I am kind of worried. Every reply will be appreciated, thank you.