I understand your impulse to help completely. I have a friend who has struggled with weight issues and body image issues and eating disorders ever since I've known her. She was pretty overweight when I first met her, then essentially stopped eating for a couple of years and became pretty underweight, and now she's on a "healthy, organic, whole-grain, vegetarian diet," and she's put on quite a bit of weight again (which in some ways is a good thing--it means she's actually eating). I would love to help her, because I know the place that she's in is dark, and frustrating, and could be reversed easily. But after getting shut down twice on the matter, I realize that arguing the point isn't conducive to the helping goal.
I agree with what a few others have said--being an example is far more persuasive than any verbal argument. I'm fit, and happy, and I maintain these things without a painful diet or a ridiculous exercise regime. Eventually, when she realizes exactly how effortless and rewarding this lifestyle is, she'll start asking questions.
(But as a sidenote, since I do love a good argument, should one present itself: To respond to the people who say that they feel awesome on their whole-grain hippie diets, you might think of the little problems that paleo fixed that you didn't even know you had. Like feeling bloated after meals, or gas, or stomach cramps, or feeling hungry all the time, or acne--things that most people think of as normal, part-of-life irritations, rather than as symptoms of a bad diet. Those are pretty persuasive points.)