Just explain that hair care is more nuanced than most people imagine - that every head of hair is different and responds differently to different products and techniques. Top it all off with a pleasant "Thanks but no thanks for the advice."
Originally Posted by Korkscrew
As with anything, I think people tend to overvalue their opinion. You're right though, every head of hair has a life of its own. What works for one should not be assumed that it will work for the other.