7-year-old is having super-soft, undigested stools

Q. I know this is an odd question, but I was asked this by a friend yesterday and couldn't answer it: is it possible to have a "sugar allergy"? My friend's 7-year-old is having super-soft, undigested stools if she eats anything with much sugar in it. My friend has tried looking at the common denominator in her diet when this happens, and it's sugar. It could be a milkshake, chocolate, or a cookie, and in the middle of the night she gets these stools and sometimes vomits as well. Have you ever heard of this kind of thing?

A. Not likely sugar as such, since she would then be allergic to herself, but the sugar can feed yeast in the gut that she could be allergic to, and also certain complex sugars such as lactose in dairy can cause trouble if a person cannot digest this (lactose intolerance).