Best place for a pint of cider in Cobble Hill area?
In addition to the other suggestions, I know the Brooklyn Inn almost always has at least one cider on tap.
You might check out Colonie, as they typically have a number of interesting draft beers. They may have cider too. Also, Bar Great Harry I think usually has one or two on tap as well.
I'm not a cider drinker either, so I don't tend to notice them.
Your best bet, as straciatella noted, is probably 61 Local. They serve ONLY local products (~200 mile radius or so), so whatever cider they have will probably be something your friend has never seen before. Failing that, Ceol will have Magner's or some other widely distributed product.
191 Smith St, Brooklyn, NY 11201
Bar Great Harry
280 Smith St, Brooklyn, NY 11231
148 Hoyt St, Brooklyn, NY 11217
127 Atlantic Ave, Brooklyn, NY 11201
61 Bergen St, Brooklyn, NY 11201
On the border of Brooklyn heights/Cobble Hill (on Atlantic) is Roebling Inn. Good selection of local, craft beers and I believe they have a hard cider on draught. Perhaps Waterfront Ale House does, as well. They have the added bonus of pretty decent pub food.
Waterfront Ale House
155 Atlantic Ave, Brooklyn, NY 11201
97 Atlantic Ave, Brooklyn, NY 11201