1. If your friend can register it in the US, that's probably the best way to go.
If Turkey is one of the officially unsupported countries, like China currently is, then it may be that Amazon won't accept a Turkish registration. So your only real option could be to pretend to be a US person temporarily living overseas in Turkey.
Another way to do it is to use the address of one of those US-based receiving/re-shipping services that sends stuff cheaper to overseas people. The sticky thread on getting a nook internationally
in the nook forum provides a lot of help and recommendations for this.
Also, buying and using gift cards to pay for books instead of your Turkish CC might be a good idea. Just in case they ever go and check whether the billing/shipping address matches up to the card.
2. You won't be able to order Kindle books for your actual Kindle without the registration.
You can still order them via Kindle for PC with appropriate VPN proxy etc., but then you'd have to strip the DRM before you could load them up on your Kindle as the files you get wouldn't be DRM-coded for the device.
PDF reading is not affected by registration status, nor are firmware upgrades. My Kindle used to be unregistered (from the device itself, due to this wireless access privacy concern I had when I first got it, so I was paranoid about turning on the Whispernet; it was registered on the website for purchasing and such) and I had no problem applying a firmware upgrade which I downloaded from Amazon's website (won't receive them wirelessly without registration) in the meantime.
3. This thread on how to register a Kindle without Whispernet access
might be useful. It's for people with a US version of the Kindle living outside the US, but some of it could probably be adapted if it turns out your friend can't do the registration, but you want to use Collections and other features.
Hope this helps.