I wouldn't recommend a cheat day. We always cheat a little over our allowed cheat. The only way I can do it is to have NO cheating. Then I only cheat a little bit.
I like the fat-burner vs. sugar-burner aspect of this. That's usually simple enough so that her eyes don't glaze over. I've been using the analogy of an electric car vs. a gasoline car. That seemed to have a good effect on my co-worker.
5'0" female, 44 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Gained back to 115(!) on SAD chocolate, potato chips, and stress. Currently keeping food tracker.
I (try to) follow by-the-book primal as advocated by Mark Sisson, except for whey powder and a bit of cream. I advocate a two-month strict adjustment for newbies. But everybody is different and should tweak Primal to their own needs.