Perhaps partially. Both she and her friend should have done their homework before going kayaking. Then they'd know that a) the area they're kayaking in is full of jellyfish, b) even if a jellyfish isn't necessarily deadly, it's rarely harmless, and c) perhaps they ought to kayak somewhere else.

It also depends on other factors. Where did Janet read that they're harmless? If it was on the back of a box of Captain Crunch (or anything published by Andrew Wakefield, though I'd trust the Captain more), then yeah, she bears more responsibility for trusting an unreliable source (though her friend perhaps ought to have asked as well). If it was in a local tourism guide or something, where the source is more trusted, it's hard to consider her responsible for it since she was clearly given poor information from an otherwise trustworthy source.