Generally, she will feel weaker in her runs for the first 2-3 weeks, then her body should have adapted. I'd recommend the book [B]Art and Science of Low Carbohydrate Performance [/B]for information on low carb athletic adaption.
Low carb can work with endurance sports - last years winner of the Western States 100 ultramarathon was a low carber.
To echo what someone else said, get her doing some strength training, especially stuff for hip drive, like squats and deadlifts. Cross-country really isn't much of an endurance thing- it's rare that the races last more than a half hour. She'll gain a great deal from getting faster.
Some ways to get faster are better than others: reaching farther forward (bad bad bad), moving the feet faster, or to push off harder, which uses mostly the muscles in the hamstring and gluteus area. Having stronger muscles there will help her cover more ground in each step without overstriding. This works well with moving the feet faster.
As far as nutrition goes, if she's 18 and still has a normal metabolism, then rice and potatoes (and maybe even some honey) are not bad things. She probably can't overdo the fruit, either. These things will help mitigate the low-carb issues that many people have to deal with. She doesn't need to do any kind of low-carb plan at all, really, if that's what works. She could also taper off the carbs slowly, to reduce the low-carb flu, but there really isn't any reason for her to be particularly low-carb.