I love a paleo meal, but having done strict paleo/primal, I know that I feel better and get better results eating cereal and ice cream. The thing to remember is that it's not all-or-nothing. You don't have to join the team and swear an oath in order to steal their food. :-) You can enjoy plenty of delicious primal cuisine and still eat a sandwich when you feel like it. Best of both worlds.
Also remember there's a honeymoon phase with any diet. God, how I went on about Body for Life, The Zone, vegetarianism, etc.. "Oh, I feel fabulous, blah, blah, blah, blood work, energy, greatest thing ever, look at my arms, never felt better, you should try it..." How many times did I do that? With how many different diets?
It's not the diet itself that makes people glow, it's the fact that they're focused on their health in a positive way, they've been consistent, and they're experiencing positive results. They could be on the packing peanuts diet and they'd corner you and gush about the wonders of styrofoam.
If your friend was a sugar fiend and experiencing the resulting mood/energy crashes and cravings, and then she started eating more meat, fat, and plants, doesn't it make total sense that she'd feel better? Not because carbs are evil but because she's getting the satisfying nutrition she needs?
Restriction backfires. Declaring something off-limits or evil is not helpful, but it doesn't mean you can't eat mostly primal or vegan or Mediterranean or French people, or whatever kind of approach appeals to you and meets your needs.
I also remind myself that people who are following strict diets, especially intermittent fasting or low carb, get some kind of a weird cortisol/adrenal thing happening where they have TONS of energy at first. I literally felt high the first time I fasted. I cleaned EVERYTHING. I danced, I skipped, I flitted, I don't think my feet touched the ground the whole day. People get hooked on that buzz, but it's a stress response. Eventually, you need to fast longer, eat less, lower carbs more, declare yourself intolerant to more foods, in order to keep feeling it. And before long you've really and truly screwed up your metabolism and hormones.
Extremes diets aren't the answer.