Assuming she takes proper care of her hair: the longer the better. However, most girls who grow their hair longer than mid-back usually end up getting dry, brittle, and frizzy ends.
Shiny, silky, mid-back hair is much more attractive than waist-length hair that is dry, brittle, and frizzy. This is why i usually place the mid-back limit when i answer these questions. However, if somehow she can maintain shiny, smooth, silky hair that is waist length - then props to her, and yeah that's probably ideal. If you don't think you can handle that, its best to play it safe.
I like long hair. The longer the better. Waist hair length is beautiful!! But I understand hair is most important to girls and it can be really hot with hair that long in the summer and realize most girls are uncomfortable with hair that long. But I think it looks amazing waist length