Definitely a good suggestion below. But you are right it may be a death sentence for the dog and unwanted conflict in your life.
A little less of a conflict may be to go to the neighbor and very mellowishly? mention that "you had the exact same kind of dog" and that "your dog died a couple years ago during the cold" (all lies of course - I hope). Maybe that will change this guys mind. Sometimes it is just a matter of perspective; especially with pets since they are so dependent on their owner's attentiveness. However, since it is a pitbull, the family is likely white trash so you may end up staring down the barrel of a shotgun in illegal possession with the serial numbers scratched off...make sure you are strapped.
What I would do, personally, is ask if the dog is tame and if the guy is cool offer to keep the dog in your garage/spare room/basement for the rest of the cold spell. Then if the owner OKs it keep it in a spare room for a couple of days, who knows? maybe you will make a new friend (dog that is,not owner)?
L.I. to Buckeye said:
Call. Seriously. It's way too cold for ANY pet or person to be outside and exposed. Pit Bulls are no longer automatically tagged as vicious. There are a number of them up for adoption at the Franklin County Dog Shelter. Unless he's a vicious dog, there shouldn't be a problem.
You could be saving the dog's life.
Here's the number for the Capital Area Humane Society's Cruelty Investigations Department: 614-777-7387 ext. 250.