I wouldn't go with a mil-surp for a sniper rifle. Mil-surps are fine as battle rifles, for taking care of torso-sized targets under 500 yards (which would mean you only need to be able to shoot 4 MOA with it), but for sniping there are a lot better choices, and you can afford to have a lot higher standards than that, even on a budget.
I would say start with a commercial hunting rifle that is fairly accurate to begin with and has a good deal of after-market support for it... a Remington 700 or a Savage 110 are the best choices IMO. You could slap any old deer rifle scope on either one of those, in any of the main calibers, and with some good fundamentals and knowing your dope, you could be VERY dangerous out to 1000 yards.
If you've settled on a Remington or a Savage, you have to pick a caliber... or rather an action length... short action or long action. Short action is .308 and all the rounds based on it, like 7mm '08, .243, .260 Rem, etc., and long action is .30-06, .300 win mag, .270, etc.
I would go with a short action, and try to find a .308 to start with. Then you would have a wide selection of good factory ammo (important for SHTF considerations, since you want to be able to stockpile a good amount of ammo, and you probably won't yet be into reloading when you are buying your starter rifle). The nice thing about the Winchester or Savage is that you can upgrade as your skills get to the point where you would benefit from it... the next things you would look at are a good barrel, trigger, and stock. You will eventually want to get into hand-loading too, since that will let you tailor a load to what shoots the very best in your rifle and is a lot more cost-effective compared to good factory ammo. You might eventually want to re-barrel in a more effective long range caliber like .260 Rem, once you get into hand-loading.
However, no matter how nice of a rig you build, a sniper rifle does not a sniper make. You definitely need to get some training. I would go to an Appleseed to begin with, because they teach the fundamentals the best of anybody I've seen and are very cheap to attend. You can go with a .22 and a brick of shells, and learn more in a weekend than just about anywhere else. In fact, I would go to the Appleseed with your buddy RIGHT NOW, before you do anything else. And then once you can shoot Expert by their standards, you can get a decent base rifle as I discussed, and look into some sort of precision rifle training near you. But if you can't even shoot Expert from field positions on Appleseed's 25m Army Qualification Test, you shouldn't harbor any illusions about being a sniper when the SHTF. And Appleseed might dis-abuse your friend of some of those fantasy notions and get him more interested in fighting the "soft war" RIGHT NOW to preserve our liberties when it's relatively easy to do so, instead of sitting around waiting for the S to HTF.
Not that there's anything wrong with putting together a good sniper rifle and learning how to use it, just in case!