Is it ok to drink coke zero on a diet? Yes and no.
Coke Zero does not contain calories, unless you believe 1 calorie is crucial for fat loss. You will still lose weight because the basic and only requirement for losing weight (caloric deficit) will not be broken with 1 calorie. There are a few problems however.
Problem 1: Coke Zero is still not a healthy choice
Think for a second. If Coke Zero has not calories what the hell is in it? Shit? Like it or not Coke Zero contains a lot of chemicals and can be harmful to your teeth, stomach and what not. According to some beliefs diets sodas are the worst because they contain acrtificial sugar known as splenda which the body cannot digest and therefor it stays in your body for years. Is it true? Well, I doubt it stays in your body for years but it sure as hell is harmful. However Coke Zero does not contain that shit but comes with a decent load of aspartame which is a dangerous substance that can cause cancer and brain damage. Whether that's true or not I don't know. If you want to know more about aspartame and it's harmful effect you can visit: http://www.aspartamekills.com/.
At the same time many people believe that diet sodas in general make you crave sugar even more due to the sweeteners and it's a dirty little trick to get the addicts back on the horse.
Problem 2: Coke Zero Can Be Addictive
Have you ever been on a diet? If so you probably know how hard it is. I believe that one of the biggest problems are the people around you for one very simple reason - they buy all this crappy food and eat it right in your face. When you are on a diet other people become drug dealers. They have all the goods you crave and you suffer. That's where Coke Zero comes in handy - it allows you to "break the habit" and cheat without cheating because there are no calories in it. However all of this can be problematic because if you drink diet sodas everyday you will still lose weight if the rest of your diet is not disrespected but you will make the diet drink a habit. Bad habit. This can be problematic. That's why I don't recommend to people to drink diet drinks in the beginning of their diets. You can add diet drinks only during the last few weeks of your diet and you should not drink more than 0.5l every two days. This is just a basic guideline that will help you in the long run.
Problem 3: Coke Zero Contains Sodium Which Causes Water Retention
It's a well known fact that diet drinks contain decent amount of sodium which causes water retention. Water retention can mask fat loss and usually blurs your muscular definition. It can be problematic. Therefor if you want to drink Coke Zero you must drink a lot of water as well to get all that sodium out of your body and avoid holding water.
Conclusion: Having a few bottles of Coke Zero at the end of your diet is ok but making it a daily habit will only cause troubles in the long run. Don't do it.