how can we show that exhaled air has more carbon dioxide
Your friend has answered correctly- the exhaled air contains carbon dioxide-this can be easily proved with the help of lime water. Take two beakers- A & B containing lime water in equal quantity. Now leave the beaker A undisturbed and blow air in the beaker B with the help of straw. It will be clearly observed that lime water in beaker B first turns milky due to high concentration of CO2 where as lime water in beaker A turns milk after taking some time due to low concentation of CO2 in the atmosphere.
@Adithya:- Good efforts, keep contributing!!