I certainly can't speak for all women, but I know for me finding out what amount of fat I could incorporate into my diet was the biggest trial and error. I love love love butter, and finding out that butter wasn't verboten on primal was like a sign from above to go nuts. I've since learned to control myself (a little) better. Nuts are another thing I went a little crazy on in the beginning. Oops.
Another thing is that calories matter, especially at the beginning when you are getting past the carb flu and maybe aren't that in tune with your body and your hunger signals, as Nameless above mentioned. I was so used to snacking all the time that I definitely over-ate primal foods in the beginning.
Finally, hormones. Those silly SlimQuick commercials with the cartoon woman talking about how she and her husband both cut out junk food and she lost 1 size while he lost 6 sizes- totally true. Men generally have an easier time losing weight than women. If she is on hormonal birth control that can also be a factor.
I don't know if there is any one thing you can do to ensure she loses weight while primal I would say just be encouraging and maybe direct her to the forums if she gets really frustrated, she is certainly not the only one who hasn't had a miraculous weight loss experience. Hopefully she is beginning to experience at least some of the other benefits to being primal, help her focus on those and not so much on the weight loss for the time being. Good luck to you both!