Today I had a heartier talk with my friend. His logic is that if a water balloon is immersed in water, then it will loose weight equal to the weight of the volume of water replaced by the balloon (which is equal to the volume of water inside the balloon). Since electronic balances determine mass based on weight, if we could somehow determine the weight of that balloon underwater, we would only get the weight of the rubber part.
Similarly, when a balloon filled with air is being weighed in normal environment, the balloon is actually immersed in air and thus Due to that Archimedes principle, weight will be lost and only the rubber part can be weighed by the Electronic balance. This is his logic.
I couldn't disagree and conceded defeat. But how can I determine the mass of air inside a balloon? Also it seems that even if a gas much heavier than air was used to inflate the balloon, we still would not get the actual mass of the gas due to buoyancy.