Question isn't geo related. Sorry for that but this is the only place I know for this stuff online.

Friends bought 2-story home with natural gas hot water heat. What can they do for central air? Girl's father said attic was the spot to put the ducts and unit in but we're in Pennsylvania so that idea sounds terrible for efficiency. Is putting a unit outside and in the basement then installing ducts on the first floor and possibly finding a spot to run the ducts through both floors and into the attic the best option? Are there other units that are a better option? Thanks.

Take a look at mini-splits. They are about as efficient as you can get and you only need smaller tubes run between the outside compressor and the in wall unit. They can heat or cool. Some also have options for a single outside compressor and 2 or 3 wall units around the home.

I 2nd the motion. There are mini-splits where one condenser can feed multiple evaporators (up to four) with only the aforementioned refrigerant tubing runs. They are whisper quiet and very efficient. Compressors vary load via incorporation of an inverter.

I ran through and eval for my aunt in NH - same situation, and concluded that a Daikin minisplit was best for her situation. Unit is also a heat pump - very cheap to operate in mild to moderately cold weather.

Mini splits are the single best way to cool this home if cost doesn't matter.
Jobs like this are very specific and designed by opportunity (closets for duct passage etc.
You need to get several contractors to your home and compare designs, don't try to decide for them.
We are doing a job like this currently. It is a big pain, but using conventional equipment I keep material and equipment cost down. I can discount volume labor as it is in my control. I can not discount volume material expense.
Routinely I find minisplits, Unico and Spacepak systems all to be 5-7 times conventional systems. They save many hours of labor, but most often cost more than the labor.
Good Luck,
Joe

There is no getting around the comfort provided by a conventionally ducted system properly designed and executed properly