No, Janet is not morally responsible (imo) because (presumably) she gave the best information she had at the time (the erroneous report that the jellyfish are harmless). Further the friend is responsible for deciding whether to swim or not in a part of the ocean with many jellyfish (which doesn't sound appealing to me) - as in why would the friend want to do this? There's no mention of what life-saving efforts Janet may or may not have taken after her friend was stung.