Lots of good feedback here. I agree with ChipperMonkey that your friend probably isn't open to much negative feedback ... she probably wants some kudos for her work and effort. So, half-truths, speed-reading to the end, and praising the good parts sound like a good compromise for both of you.

BTW, a friend asked me to the same thing several years ago. The book he wrote was also flawed but I fudged my feedback and we remained friends. He didn't press me very hard on my input ... he really just wanted to feel proud of his efforts.