Nancy wants to know how to keep a thread straight on Gmail. If she deletes an email, she deletes the thread? Leo says it depends on settings, but it's called "conversation view." Nancy can turn the threading off if she wants. This article on Lifehacker shows how to delete a reply in a thread. This may be different in the iPad, though.
Leo suggests expanding all threads and then see if she can delete individual ones. Leo doesn't like threading because it's so confusing for issues like this -- especially on the iPad. Nancy should turn off the "Optimize by Thread" setting in the iPad's Gmail settings. That will make it much easier to wade through.