[For those of you on the metric system, an ounce is about 29.6 milliliters and a teaspoon is about 5 milliliters]
This is hard to believe, but neither of you is right. There's actually more sugar than your friend suggests! Here are two ways to prove it to yourself.
The first is to buy a packet Kool-aid or Flavor-aid "unsweetened soft drink mix" -- that is, the kind you add sugar to when you are making it. You will be instructed to add one cup of sugar and enough water to make two quarts (64 ounces). A cup of sugar contains 48 teaspoons of sugar. Therefore, a 16-ounce serving of one of these beverages contains 12 teaspoons of sugar!
The other way to prove it is to look at the calorie count of any soft drink. For example, a typical carbonated soft drink will have 200 calories in a 16-ounce serving. All of those calories come from sugar, and sugar contains 16 calories per teaspoon. By this measurement, a 16-ounce serving contains 12.5 teaspoons of sugar.
So go down to the kitchen and get out a 16-ounce glass, a teaspoon and some sugar. Measure 12 teaspoons of sugar into the glass -- it's an amazing amount! Then multiply that by however many sodas you typically consume in a day.
Another thing they put in many soft drinks is caffeine. See How Caffeine Works for details.
Here are several interesting links: