I agree. Turtles are not appropriate for children.
1. Children have short attention spans. She may want one now but what about when she moves on to her next fixation?
2. Turtles require a lot of care. Especially aquatic turtles. Your boyfriend can count on doing all the work.
3. Turtles bite.
4. Turtles are not like domestic pets. They aren't lovable and they just generally don't make good pets for someone who wants to handle them often. They are better off just being looked at.
5. Even if she does decide to stick with the turtle and she takes care of it and so forth it will still be around when she is ready to move on to college. Turtles are extremely long-lived when they are cared for properly. What would happen to it when she moves on in her life?
6. Most aquatic species get big. When they are full grown they require housing outdoors. You have to be prepared to build and maintain a pond for the duration of the turtle's life.
I recommend a hamster or something equally cute yet relatively short-lived. Their maintenance is comparatively low and they are great pets for teaching responsibility.