What is the carbon footprint of a new bike?
I don't have numbers on hand. But if you're after a cogent argument, I would point out that the car they drive has a far larger manufacturing footprint, even assuming it is manufactured in the local economy, which is rare. If it is, then was the raw steel manufactured there or shipped in? 

For that argument of theirs to be effective, they must start with the idea that their car popped out of thin air. Even assuming that the manufacturing footprint is identical, the car continues to build that carbon debt, while the bicycle mostly ceases to charge that card after the manufacturing is done. 

Of course there will be minor maintenance items, and consumables like tires and tubes, but the car has those, too. No matter how you look at it, a bicycle doesn't burn fuel to function and a car does.

So the car will use far more resources over the course of it's useful life.

Also, whether you believe that global warming is real and important, or not, it will affect your finances far more to drive a car than to ride a bike.