Hmmm, think he'll need a bit more than the intended 3 days for the AT.........
Cranberry Lake region is nice. The new 50 Trail is nice, could easily park a vehicle in a couple strategic locations to make a nice through 3 day hike. Decent places to camp, either tent or a few leantos along the way. Some sweet fishing in certain locations. Not a lot of vertical, so perfect for someone or small group to do as a warmup. Blackflies can be down right oppressive if you time your trip during the prime hatch though.
I also like the northern section of the Northvillle Placid trail, very nice. Get a taste of the High Peaks w/o a lot of vertical. I personally would not suggest the Eastern Zone of the High Peaks (Marcy/Colden/Algonquin area) for a multiday trip for a newish group to this kind of thing. Do a few day trips in there first, during an off-peak weekday(s). Trust me on this. Excessive amounts of poorly prepared people, restrictive camping regs (read no fires), most of these trails go straight up the fall line in places(read steep) and several other reasons come to mind....
Once you get more accustomed to what you like, what you can do in a day and what is out there to explore you could spend years hiking the 'Daks w/o seeing the same sights twice.
The FLT is also a good option. Centrally located for your friends and plenty of bail outs in case someone wants an out. FLT does indeed cross much private lands that are closed to camping but it also zig-zags to touch virtually every piece of State Forest along the way, which are open to primitive camping. A very little research a head of time and you could plan reasonable camping/overnight spots and also stops to acquire provisions while on your trek. Maybe consider a section through Cortland/Tompkins counties.
Good luck.