If you focus on your “comparative advantage” of printing paper and your friend on his comparative advantage of making stuff, then in the long run he’ll win because your economy is fake and his is real.

What does he win? More paper? Less glibly - What does a short term tariff on the goods I’m buying from him do that isn’t already baked in to this “long run” scenario you’re envisioning? Say I mark up all the goods I buy from him and the result is that I have to spend more time making goods - why wouldn’t I just do without tariffs, benefit from as many goods as possible, and then start making more goods myself whenever my neighbour stops accepting my paper?

It is simply because the USA consumes more.