From your description, and without a picture, probably not purebred. A white chest is sometimes possible with a black/tan GSD, but paws are pushing it, and the tail should never have a white tip.
I would also never pay someone to help them and take their dog off their hands. If they want it to have a good home, I'll give that dog a good home (especially since you're being recommended by a friend), but I'm not about to pay you for a dog that you did nothing for. This is a lesson learned and not a way to recoup the costs of a dog.
You also have no way of knowing how much she paid for the dog, what kind of veterinary care he's gotten, and what if she does this on a regular basis, makes up some sob story and then makes a profit off of a shelter or rescue dog.