It could well be true, though distinguishing the cause of her symptoms - whether it is an allergic reaction (i.e. the immune system inappropriately overreacting

It could well be true, though distinguishing the cause of her symptoms - whether it is an allergic reaction (i.e. the immune system inappropriately overreacting), problems in absorbing or metabolising fructose (so primarily related to intestines or liver), or something else - would be difficult without the involvement of a medical professional.