I think the “pushy Asian parents” have shown us that it’s not all about innate talent. If you study hard you can accomplish a lot more than you might think. As others have said, we don’t know if your friend needed that summer studying, or if she’d have done fine without it. My kid would be the first to tell you he learned a lot of his history from reading all the background info that came with computer games like Civ 4. But he was reading about at least military history avidly since he was in first grade. He also had a real ability to write essays on the fly. Many in his AP World class went into a panic when the main DBQ essay was about the first modern Olympics (which they had not studied in class needless to say!) Having that background was a big help, as was realizing the mindset of the people putting together the exam. ("They probably want me to write about Nationalism Internationalism and all the other issues that led to WW1.)
I don’t think reading a review book is a bad way to start an AP course - it gives you a good idea about the salient points which you will study in greater depth over the course of the year. Here in NY where AP kids are shortchanged a month or so of school compared to other parts of the US, reading a review book is a common summer assignment for AP classes.