My stylist actually recommends that I change shampoos every once in a while. The reasoning is that your hair becomes used to the particular shampoo and looses a bit of it effectiveness.

I mix shampoos, I generally use moisturizing but I do use a clarifying shampoo for a week or so every couple of months.

Did your friend say why you shouldnt use different shampoos?

"Diplomacy is the art of saying 'Nice doggie' until you can find a rock - Will Rogers"