Hi,
I would look in predominantly Hispanic or Latin neighborhoods, for grocery stores in their communities. I am not too familiar with NY city, so can't give you a particular area or Grocery store name, but if your friend can find a grocery store that sells Mexican/Hispanic, or Latin groceries, they are sure to carry Nopales.
Here in Texas, Nopales, come in whole pads with the thorns, that you have to clean before you cook. Also you can get whole pads, with the thorns cleaned for you. And now most stores also carry Nopales that are already cleaned and diced ready to cook.
I am certain that if your friend asks around in that community, they will be readily available. Ask them to look up Latin groceries in the yellow pages and call the store to ask for Nopales. ..
Even if they don't have them, they'll direct you to a supplier.
I hope this helps and that your friend can locate them, but if not, just leave me a note. They are available here, and I can mail some to them.
Good Luck..