I have a friend who's dog would respond to a command by a stranger, sit, down, etc...what went wrong in training, and how can it be corrected and/or prevented?

Sounds like a well behaved dog to me. Is this dog just a pet? Is there reason to worry that someone could steal the dog? A well trained pet dog who is not wary of strangers, will many times respond to commands from others. The key is that the dog should respond first to his master and if the dog must choose then it should always choose its master. My Lab is very skeptical of strangers and would not be quick to listen to a command from a stranger. My Golden Retriever would just about go with anyone. With this breed it is pretty common, they just love everyone. Before my retrievers I had a GSD. I didn't have to worry about him getting stolen! Believe me!LOL. If this is the concern than I understand your question. Are you talking about a stranger that is invited in the house or has met the dog for the first time? If this is so, then the dog has taken his cue from his pack leader(owner)and has accepted this "stranger" into the home. It depends on your definition of a "stranger". Stranger to you or the dog or both? Again if your fear is that the dog could be stolen then I know what you mean. Other than that, if we are talking about a pet dog then it sounds like a nice dog and I'm not real sure what the problem is.

Thanks for your answer I'll pass it on to him. His dog is a pet.

One command a pet should listen to its owner over a stranger is the COME. You work this by both you and a stranger approaching your dog letting him smell food in your hands while nothing in the strangers. Then move away from the dog and both of you call him. If he starts going to the stranger show him the food visibly. Then repeat this with both of you having food but you always have the tastier food. This works great. I go one step further by having a stranger pet my dog and when I call him the stranger stops petting and looks away from the dog ignoring him. It gives the impressing that the whole world stops when I command my dog and more importantly that what’s with me is more interesting than what is over there.

Good advice Vince. I definately agree.