Can the Acer run OS X and the full spectrum of OS X applications?
That's just proving his point more. There are no apps that lock you down to OS X. People can switch back and forth with ease.
However, I can find you a big hulking desktop that beats his laptop choice on specs and price too, showing him his laptop is also "overpriced".
It's easy to find cheaper and "better" when you don't take into consideration the whole package. The Air's form factor is very much a feature and you very much pay for it. Compare other ultra portables like the Sony Vaio X and the Dell Adamo and you notice the Air is not that expensive after all.
Of course if you start comparing the Air with full tower desktops, you might not understand where your money is going, but I sure wouldn't want to dragging and using a full tower desktop on the bus.