No, this dog did not have rabies.
This dog was very sick from not being taken care of properly, and died from neglect.
Any number of things can kill a puppy. This is why you get them vaccinated. This is why you take them to the vet when they look sick.
You have also exposed your dogs to diseases every time you have gone over to their house, or any other house with a dog that isn't vaccinated. If your dogs are not vaccinated, not only are you breaking the law, but you are encouraging them to die a horrible, painful death like your friend's dog did.
If your friend wants a necropsy done, he can. Dig the body up, take it to the nearest vet that offers this, and hope they don't call the police on your friend. Like I said, this dog died of something preventable, this is a criminal act, as well as not having him vaccinated. If he does get a necropsy done, the police will probably be waiting for him when he comes to collect the ashes.