That is so sad. Most dog behaviorists say never to leave a dog with kids alone -- children are unpredictable and apt to startle even the most good-natured dog. Moreover, there's a good chance your friend's dog was being protective of the new baby. Read Jan Fennell's "the Dog Listener" which points out that, in the absence of the human being a strong alpha, even a timid dog feels it has to become pack leader and this can stress it out and lead to all sorts of problems. Wish they could have rehomed it.