Ask them what backs their government issued FIAT, its one private corporation. With Bitcoin it is collectively backed by the miners and you would rather trust something that has a real democracy behind it that is not at the mercy of the greed of a select few individuals. Offer them a challenge, Keep 100 dollars worth of Bitcoin (at the current rate) and a 100 dollar bill for 10 years then see which has the most purchasing power after the 10 years has passed. If they don't want to take the challenge they clearly don't believe in practicing what they preach.
Havent tried that one yet! Minus saying more individuals back bitcoin than USD and too many people have jobs based on bit coin dev, mining, invested etc, for it to suddenly crash to $0 in a day like it could a few months ago, albeit it still can, more people would resist the price drop and it would surge in price again if that DID happen.