All sorts of things can cause hair loss, from allergies to thyroid imbalances.
Can you tell us a little more about her background? How old is she? Where did you get her?
If food allergies are suspected, a switch to Royal Canin wouldn't be a good idea because it's full of corn which is known to cause allergies. Are you sure he wasn't recommending Waltham, their prescription brand?
Not sure what not being purebred would have to do with hair loss....
