There's nothing logical at all about any of this, and I'm not even getting into the whole zombie thing. I mean, do you realize how utterly impossible it would be for virtually any country or force to take over the United States?
There's really only one country, perhaps two, that could even dream about taking this country over from a military standpoint. China, and maybe Britain if they had most of Western Europe with them and they got the drop on us with some Kim Philby-type of espionage that blinds us to a surprise attack.
But that'll never happen. China could never go to war with us in the next ten years. They JUST got their first aircraft carrier up and running and quite frankly, they'd had have MUCH more to lose than to gain if they attacked us. Our economy would necessarily fall apart within days if China launched an all-out assault that actually managed to succeed. That means that theirs would be right behind ours, along with most of the civilized world. The reality is that the only countries with any interest in wiping us off the face of the Earth will never have the capability of doing so and are so destitute that they would only minimally be harmed by the utterly complete collapse of the entire world's economic system.
Now, can Iran potentially get a nuke within 10 years? Of course, but how are they going to deliver it here? They aren't, and they don't have the sort of standing army, or the means of moving them across a continent and an ocean, in order to back up this theoretical nuclear strike. If Ahmadinejad is really as wacky as the chickenhawks in Washington think he is, he'd just deliver it to Israel, kill about 200,000 people and then Israel would use their nukes to turn Tehran into glass.
Same thing with North Korea. They can't get an ICBM across the Sea of Japan so how are they going to deliver a huge payload across the entire Pacific? And even though they are the only country where there is even a remote possibility that their leaders would entertain a full-out attack of the U.S. in their darker moments, they are also the only one with a large enough standing army to be of any threat to the U.S. Except that their army is significantly malnourished and lacks an effective way of transporting that many troops across the Atlantic in a manner that we couldn't stop with, say, one or two F-22 Raptors. And even if they COULD do all of this, they wouldn't do it us, they'd do it to South Korea then China would step in and probably intercede on behalf of South Korea.
Regarding famine: there are breakthroughs being made in the wide world of genetically-engineered food everyday. On top of that, one positive to accelerated climate change is that there are all sorts of new areas opening up to agriculture that was never available before. A famine in one area may simply mean a large boost in production in another area.
From a purely social standpoint, within the U.S., I don't see it happening. I think this is the most likely scenario, but it isn't likely at all. Who's going to lead this societal revolution? Hardcore liberals? They failed miserably in the sixties already. Hardcore conservatives? They won't take over, they'll just check out and turn into survivalists. Anarchists? I don't think so. They're too in the minority and they'd have the full weight of the federal, state and local govts weighing down on top of them the second they got too far out of line. And the rest of society just simply Does Not Give a Fuck.
So I don't think there is a single shred of logic in your doomsayer friend's thinking. None whatsoever. It sounds to me like he hasn't directed one logical, cogent thought toward this scenario at all.