A 12k trailer does not necessarily constitute the need for a dually. If you want one, I can understand that, but you are going to pay up front and then pay as you go for tires and maintenance. Tires for my F350 set me back $1200, vs. $700 for my F250. Tire wear is not as good because they are more difficult to rotate. Both NY & PA assess higher registration fees for dually's and most garages charge more for inspection and other service. It is TOTALLY up to you! A 2500 will easily handle your trailer and tractor. (I assume your trailer is a conventional tow rather than a gooseneck.) Dually or single rear wheel, you are going to need a weight distribution hitch. You should be able to find a hitch rated for 12,500/1250 which should exceed your need.
I checked the local market here and didn't find much in your price range. You are better of lucking in the south anyway to avoid road salt rust! Good luck and happy shopping.
Why would he need a weight distribution hitch?