Is zero diet cola good or bad for someone's health ?

Neither good/bad: Several recent studies have found that non-nutritive sweeteners were a useful adjunct to weight loss regimens when used in moderation. This is a general rule that applies to many foods, treatments, and food additives. So, to answer your question zero diet cola is NOT harmful, if not used as the ONLY beverage in consuming your daily meals and snacks..,

Zero cola: cola, whether diet, zero or regular, is not good for your health. It tends to stimulate your pancreas increase the risk of diabetes in the future