Greetings from the UK. I have a friend going to NYC next week. They have promised to pick me up some beers. Any suggestions, either NYC beers or American beers that they should be able to find.
New York state, as a whole, has pretty good beer coverage. I can get most brands. I currently live in Western NY.
The good thing about NYC is that you can find just about anything. Brooklyn Brewery is good, but I'd ask for some West Coast IPAs like Stone, Firestone Walker, or Lagunitas as well. Also, maybe some East coast beers from Dogfish Head (weird stuff), Omegang (belgians), Southern Tier (anything), or Troegs (anything). http://newyork.cbslocal.com/top-lists/nycs-5-best-craft-beer-stores/.
This site will show you which breweries distribute in New York - http://www.seekabrew.com/distro/index.html. You won't be able to find everything at every bottle shop, but you can see what's theoretically available. It might help you put together a list of what you want.
I agree with all of the above --- although a lot of these brands are seeing increased distribution and are easier to find elsewhere (throughout country in stores/bars etc.). That said for your buddy I recommend the following.
They have about 15-20 taps and even as a beer fanatic I rarely see anything on tap that I recognize (they get VERY impressive kegs). They have good bar snacks and there are plenty of bodegas near by where your buddy can pick up most if not all of the suggested beers in this thread. Want something more exotic? Ask the bartender --- they are extremely knowledgeable.
I recommend this route because it would be a Great experience --- Exploring stuff in NYC off the beaten path from the typical tourist areas (and an area that will give you a whole new perspective on living in NYC) and an easy jaunt from Manhattan...