I refer to your fasting friend who's decision to have diet coke, while fasting (and while eating), is not well informed.
citation form Yahoo health:
"Myth 4: Artificial sweeteners are better than sugar
Truth: "Gas, bloating and diarrhea are often linked to artificial sweeteners, such as sorbitol and xylitol," says Calgary-based registered dietician Sarah Remmer. "I encourage my clients with digestive complaints to avoid them." Some research in animals also shows that artificial sweeteners may cause you to eat more calories, which further derails your diet. Researchers suspect that when sweetener touches your lips, your brain sends a signal to your stomach that Sugar is coming. When none arrives, this can result in stronger cravings later, making you more likely to reach for chocolate come 3 p.m. If you need sweet stuff, put a dash of Sugar in tea or coffee. "