I knew a guy that tried this with his new rottweiler. She was only 40lbs when he got her and really young. He kept doing this and she slowly got more aggressive, even as I was telling him he was making things worse.. Finally when she realized she was big enough and strong enough (95lbs) she mauled him.. he was hospitalized and she was euthanized. I know there's a huge size difference between a rott and a JRT, but the result is the same...
My dog was pretty aggressive toward anyone when I got him 8 weeks ago.
What we did is whenever he bit, we very *calmly and sternly* told him no and proceeded with what we were trying to do (normally remove something he shouldn't have from his mouth or get him off the couch). If he tried to bite or growl again we used a plant mister to spray him once with water. That distracted him long enough to get ahold of his collar and snap his leash on without getting bitten. Then we put him through obedience exercises for a few minutes, always giving treats and praising like crazy when he did what was asked of him.
If the dog has gotten used to biting though that could be dangerous because if he's on his leash he can still get to you. I would talk to a vet or find a behaviourist. My dog was only 9 months old when we got him (we think, don't know for sure) and the vet, who is a behaviourist, said that if he had been allowed to continue that behaviour another couple of months he probably never would have been safe to be around. Time and age are the keys to aggression, so I've learned.
In any case, try to convince your friend that this tactic is probably making things worse and that he and the dog really need to seek professional help. I made the mistake at first of thinking I could handle my dog's aggression on my own and I was dead wrong, professional guidance is the only way this will get better.
When he starts biting his owner, it's time for a major change in tactics..