The method will work best if your friend is strong enough to actually change the dog's momentum--if the dog is powerful enough to pull her over, it may not work.
If she can find a safe fenced-in area to start, it will be safer.
She walks the dog forward and the very second the dog moves out ahead and the leash starts to get taut, she does a 180 and walks off in the opposite direction, tugging the dog with her. Ideally, the dog stops and comes along, too. Every time the dog forges ahead to pull, she changes direction randomly, bringing the dog along. Eventually, the dog will begin to learn that it has to watch its owner and follow along.
It does work after a while but all the pivoting and direction changes can be dizzying till the dog gets the idea.