My turtle's have ate a scute of their own shell before. I honestly dont know if it is normal or whatnot. But this was a long time ago, and they are fine now. Plus, I think if it was harmful more people would warn other's about the danger of it (just like gravel) but I have never heard of such a thing.
Red leaf lettuce is good. My picky eater prefer's it over green leaf

Remember to add more than just red leaf lettuce, when your done with the head of red leaf lettuce, buy green leaf instead, and throw in some thin strip's of carrot's. Aquatic plant's are good too. Anacharis, hornwort and duckweed is good.
You can give her a nice gutloaded cricket or worm once every two week's too.