It's not about who's responsible for checks, that is academic. The layman has asked the expert a question. The expert has given the wrong answer, the layman has taken the advice in good spirit and bought a product. That's all there is to it mate and all a judge will see. It's crap and a sign of the arse covering world we live in but men are no longer men and stand by their own decisions and responsibilities.
The legal standpoint is clear, everything else is just down to how quickly and easily you can put this situation to bed and move on. Good luck.