If it is a young puppy and a rather foul diarrhea, you should rush it to a vet even without contacting the owners. Otherwise, in the sticky at
http://www.askmehelpdesk.com/dogs/in...tml#post254171 there is a recipe for a bland diet mean to settle digestive systems. Yogurt is a popular addition to a dog's diet, but better vets don't seem to suggest it. I think much of the push for it comes from the invalid idea that if it is good for people, it is good for dogs.
What is good for dogs is a kibble and little else, as long as they will eat it.